migration: Fix block migration flag case

migration: Fix block migration flag case

Fix the case where when a migration with a bad protocol is tried,
we leave the block migration capability set.

(This is a cut down version of my 'migration: Fix block failure cases'
where it's other case was fixed by Peter's dd0ee30caeebbd )

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20180316202114.32345-1-dgilbert@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>