This migrates the iso-live-login tests from the standalone 'kola testiso'
command into the standard 'kola run' framework, making them available
alongside other kola tests.

Changes:
- Moved iso-live-login* tests implementation from cmd/kola/testiso.go to
  mantle/kola/tests/iso/live-login.go
- Removed 4k test variants (iso-live-login.4k.uefi) as they were
  non-functional for this test type

Why 4k tests were dropped:
The 4k flag in testiso only affects disk sector size during disk creation.
However, the live-login test boots directly from ISO without creating any
disks, so the 4k flag had no effect. The 4k test variants were redundant
and ran identical code to their non-4k counterparts.

Relocate metal.go to machine/qemu/ directory where it belongs, as it
contains QEMU-specific code for ISO installations and is only used by
ISO tests, not as a general platform implementation.

Temporarily consolidate InstalledMachine into machine struct to simplify
the codebase during the testiso migration. This prepares for migrating ISO
installation tests from the testiso command into the standard 'kola run'
test framework.

This merge is temporary and will be refactored once the migration is complete.

Split the monolithic live-iso.go file into several focused files where
each file contains a specific set of tests:

- live-pxe.go: PXE network boot tests
- live-iscsi.go: iSCSI boot and installation tests
- live-as-disk.go: ISO-as-disk installation tests
- common.go: Shared test utilities and helper functions

This improves code organization and makes it easier to locate and
maintain specific test suites.

Add journal dumping to signalFailureUnit to capture diagnostic
information when the system enters emergency.target during ISO tests.

After switch root occurs during live ISO testing, coreos-installer runs
from the real root filesystem rather than the initramfs, which means
ignition-virtio-dump-journal.service is no longer enabled. This change
ensures that when emergency.target is reached, the journal is still
dumped to the virtio port, allowing platform.StartMachine to capture errors.

The unit now checks if ignition-virtio-dump-journal.service is enabled,
and if not, manually executes the journal dump script from the dracut
emergency shell setup module.

Move the 'inst-insecure' and 'pxe-kargs' parameters from local test options
to global QEMUOptions structure, making them available as command-line
flags for all ISO tests:

- --inst-insecure: Skip signature verification on metal image
- --pxe-kargs: Additional kernel arguments for PXE boot tests

Migrate the last remaining test from the legacy 'kola testiso'
command into the standard 'kola run' This completes the migration
of all ISO tests and removes the now-obsolete testiso.go file.

Extract duplicate completion channel reading logic into a reusable
CheckTestOutput helper function. This reduces code duplication across
iso tests.

The kola test harness already performs console and journal checks
automatically after each test completes, making the duplicate checks
in awaitCompletion unnecessary.

Call flow in mantle/kola/harness.go:
```
  runProvidedTests
    -> runTest
         -> handleConsoleChecks("console", ...)
              -> CheckConsole
         -> handleConsoleChecks("journal", ...)
              -> CheckConsole
```

Update the test to use the existing MachineBuilder API after the
testiso migration.

Also remove the 4k sector size test variant as it's technically impossible
to configure. The AddIso() function and iso-as-disk implementation do
not support sector size configuration - the ISO is attached as a raw,
readonly device without any sector size options.

The Disk.SectorSize field only applies to regular disk images added
via AddDisk() or AddDisksFromSpecs(), not to ISOs attached via AddIso().
Since iso-as-disk tests boot directly from the ISO file (not a disk
image), sector size configuration is not applicable.

Only wait for SSH address when usermode networking is enabled.
This allows ISO tests without networking to proceed without
unnecessary SSH address waiting.

Add Instance() method to expose the underlying QemuInstance from
a platform.Machine. This allows tests to access QEMU-specific
functionality not available through the generic interface.

Extract the Ignition config path logic into a reusable IgnitionPath()
method that returns the path where the Ignition config will be written.

This allows callers to get the Ignition config path before rendering,
which is useful for tests that need to create symlinks or perform other
operations on the config file path before the machine is started.

Allow callers to pass nil userdata/config when Ignition configuration
is not needed, such as in PXE boot scenarios where configuration is
provided through alternative means.

Remove deprecated machine struct fields and methods that are no longer
needed after the migration all ISO tests under kola.

Replace enableUefi and enableUefiSecure boolean flags with a single
firmware string field in IsoTestOpts for simpler and more consistent
firmware configuration across all ISO tests.