KVM leaves conversion XML and UEFI NVRAM files behind #14462

Migration testing found two cleanup problems:

• After a cold import directly into NFS primary storage (forceconverttopool=true), the temporary <conversion-uuid>.xml file remains, including after the VM is expunged.
• After a UEFI VM is expunged, its <vm-uuid>.fd firmware file remains on the KVM host.

The tested migrations and guest checks passed. These leftover files did not block the tests, but required manual cleanup.

Both cleanup gaps predate PR14256:

• Conversion XML: PR11594, commit b106d6e.
  b106d6e
• UEFI NVRAM lifecycle: PR3638, commit d4b537e.
  d4b537e

Expected behavior: remove unused conversion XML after successful import, without deleting the adopted disks; remove unused per-VM NVRAM after permanent expunge, not during stop/reboot or recoverable destroy.

Found while testing PR14256: #14256