migration: savevm: fix error code with migration blockers

The only caller that checks the error code is looking for != 0,
so returning false is incorrect.

Fixes: 5aaac467938 "migration: savevm: consult migration blockers"

Signed-off-by: Cole Robinson <crobinso@redhat.com>
Message-Id: <b991a4d0e6c4253bc08b2794c6084be55fc72e1d.1554851834.git.crobinso@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

migration/savevm.c

@@ -2542,7 +2542,7 @@ int save_snapshot(const char *name, Error **errp)
     AioContext *aio_context;
 
     if (migration_is_blocked(errp)) {
-        return false;
+        return ret;
     }
 
     if (!replay_can_snapshot()) {