Don't require secdrivers to implement .domainMoveImageMetadata

Don't require secdrivers to implement .domainMoveImageMetadata

The AppArmor secdriver does not use labels to grant access to
resources. Therefore, it doesn't use XATTRs and hence it lacks
implementation of .domainMoveImageMetadata callback. This leads
to a harmless but needless error message appearing in the logs:

  virSecurityManagerMoveImageMetadata:476 : this function is not
  supported by the connection driver: virSecurityManagerMoveImageMetadata

Closes: https://gitlab.com/libvirt/libvirt/-/issues/25
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>