backup: fix handling labels in Qubes 4.0 backups
backup: fix handling labels in Qubes 4.0 backups

Labels can be referred as either id or name. Support both ways.

Fixes QubesOS/qubes-issues#3211

marmarek/qubes-core-mgmt-client,marmarek/qubes-core-mgmt-client,marmarek/qubes-core-mgmt-client