virsh: fix snapshot-create with no xmlfile

Properly check the return value of vshCommandOptStringReq for xmlfile:
* error out on incorrect input (--xmlfile '')
* use default XML <domainsnapshot/> with no --xmlfile specified

(Broken by commit b2e85855)

Bug: https://bugzilla.redhat.com/show_bug.cgi?id=919826

tools/virsh-snapshot.c

@@ -211,7 +211,9 @@ cmdSnapshotCreate(vshControl *ctl, const vshCmd *cmd)
     if (!(dom = vshCommandOptDomain(ctl, cmd, NULL)))
         goto cleanup;
 
-    if (vshCommandOptStringReq(ctl, cmd, "xmlfile", &from) < 0) {
+    if (vshCommandOptStringReq(ctl, cmd, "xmlfile", &from) < 0)
+        goto cleanup;
+    if (!from) {
         buffer = vshStrdup(ctl, "<domainsnapshot/>");
     } else {
         if (virFileReadAll(from, VSH_MAX_XML_FILE, &buffer) < 0) {