qemu: Don't return success if creation of snapshot save file fails

When the snapshot definition can't be saved, the
qemuDomainSnapshotCreate function succeeded without filling some of the
fields in the internal definition.

This patch removes the snapshot and returns failure if the XML file
cannot be written.

src/qemu/qemu_driver.c

@@ -11598,8 +11598,14 @@ cleanup:
         if (snapshot && !(flags & VIR_DOMAIN_SNAPSHOT_CREATE_NO_METADATA)) {
             if (qemuDomainSnapshotWriteMetadata(vm, snap,
                                                 driver->snapshotDir) < 0) {
-                VIR_WARN("unable to save metadata for snapshot %s",
-                         snap->def->name);
+                /* if writing of metadata fails, error out rather than trying
+                 * to silently carry on  without completing the snapshot */
+                virDomainSnapshotFree(snapshot);
+                snapshot = NULL;
+                virReportError(VIR_ERR_INTERNAL_ERROR,
+                               _("unable to save metadata for snapshot %s"),
+                               snap->def->name);
+                virDomainSnapshotObjListRemove(vm->snapshots, snap);
             } else {
                 if (update_current)
                     vm->current_snapshot = snap;