snapshot: fix regression with system checkpoints

Regression introduced in commit d6f6b2d1.  Running
'virsh snapshot-create dom' would mistakenly report that
disks can only be specified for disk snapshots.

* src/conf/domain_conf.c (virDomainSnapshotDefParseString): Only
give error about no disk support when <disk> was found.

src/conf/domain_conf.c

@@ -11622,7 +11622,7 @@ virDomainSnapshotDefParseString(const char *xmlStr,
                 goto cleanup;
         }
         VIR_FREE(nodes);
-    } else {
+    } else if (i) {
         virDomainReportError(VIR_ERR_ARGUMENT_UNSUPPORTED, "%s",
                              _("unable to handle disk requests in snapshot"));
         goto cleanup;