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 b2e8585)
Bug: https://bugzilla.redhat.com/show_bug.cgi?id=919826
(cherry picked from commit 2fc5ff11ab796a8ad60e0432373c93d0794fbfc6)