migration: check global caps for validity

Checks validity for all the capabilities that we enabled with command
line.

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Message-Id: <1500349150-13240-11-git-send-email-peterx@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
