core: change failed migration audit logs to errors

The audit logs VM_MIGRATION_ABORT and
VM_MIGRATION_FAILED_NO_VDS_TO_RUN_ON were WARNINGs instead of ERRORs

Change-Id: Ibff323fc109b326010844d63b296c9d9ab6155a0
Signed-off-by: Arik Hadas <ahadas@redhat.com>