Fix memory leak in cmdAttachDisk

When virBufferError is ok in cmdAttachDisk, the latter
should 'goto cleanup', instead of returning a false to
prevent memory leaking.

Signed-off-by: Eric Blake <eblake@redhat.com>

tools/virsh-domain.c

@@ -661,7 +661,7 @@ cmdAttachDisk(vshControl *ctl, const vshCmd *cmd)
 
     if (virBufferError(&buf)) {
         vshPrint(ctl, "%s", _("Failed to allocate XML buffer"));
-        return false;
+        goto cleanup;
     }
 
     xml = virBufferContentAndReset(&buf);