migration: fix handling for --only-migratable

MigrateState object is not ready at that time, so we'll get an
assertion. Use qemu_global_option() instead.

Reported-by: Eduardo Habkost <ehabkost@redhat.com>
Suggested-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Fixes: 3df663e5 ("migration: move only_migratable to MigrationState")
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <1499242883-2184-2-git-send-email-peterx@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>