migration: fix applying wrong capabilities

When setting migration capabilities via QMP/HMP, we'll apply them even
if the capability check failed.  Fix it.

Fixes: 4a84214e ("migration: provide migrate_caps_check()", 2017-07-18)
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180305094938.31374-1-peterx@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>