tests: add multipath.single-disk to verify that multipath can be reduced to one path

[HuijingHei]

Reduce disk on multipath via qmp, then reboot.
{ "execute": "device_del", "arguments": { "id": "/machine/peripheral-anon/device[3]"}}

See https://issues.redhat.com/browse/OCPBUGS-56597

[gemini-code-assist]

Code Review

This pull request adds a new test case, multipath.single-disk, to verify that a multipath device can be correctly reduced to a single path after one of its underlying block devices is removed. The changes include a new test function, modifications to the QEMU platform code to allow for block device removal, and associated test setup. My review focuses on the robustness of the new test logic and code style consistency. I've identified a potential bug in parsing command output within the test and a style inconsistency in one of the new functions.

[HuijingHei]

Confirm with @ravanelli , the failed CI ci/prow/rhcos is blocked by https://issues.redhat.com/browse/RHEL-126696

=== RUN   multipath.single-disk
--- FAIL: multipath.single-disk (667.10s)
        multipath.go:273: Failed to reboot the machine: machine "b56daba4-bd95-4bfb-a151-964c2619c032" failed to start: ssh journalctl failed: time limit exceeded

[HuijingHei]

@ravanelli could you help to review this, as the new coreos-installer is synced to 4.19 plashet now, thank you!