virsh: fix snapshot-create with no xmlfile
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 b2e8585)

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

fabianfreyer/libvirt,zhlcindy/libvirt-1.1.4-maintain,nertpinx/libvirt,zhlcindy/libvirt-1.1.4-maintain,rlaager/libvirt,zippy2/libvirt,datto/libvirt,taget/libvirt,elmarco/libvirt,agx/libvirt,datto/libvirt,crobinso/libvirt,shugaoye/libvirt,rlaager/libvirt,andreabolognani/libvirt,zhlcindy/libvirt-1.1.4-maintain,jfehlig/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,VenkatDatta/libvirt,eskultety/libvirt,cbosdo/libvirt,jardasgit/libvirt,VenkatDatta/libvirt,datto/libvirt,rlaager/libvirt,taget/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,agx/libvirt,nertpinx/libvirt,datto/libvirt,rlaager/libvirt,cbosdo/libvirt,andreabolognani/libvirt,libvirt/libvirt,VenkatDatta/libvirt,libvirt/libvirt,cbosdo/libvirt,shugaoye/libvirt,agx/libvirt,libvirt/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,taget/libvirt,andreabolognani/libvirt,olafhering/libvirt,eskultety/libvirt,olafhering/libvirt,jfehlig/libvirt,zippy2/libvirt,taget/libvirt,eskultety/libvirt,andreabolognani/libvirt,agx/libvirt,jardasgit/libvirt,cbosdo/libvirt,crobinso/libvirt,eskultety/libvirt,nertpinx/libvirt,elmarco/libvirt,crobinso/libvirt,elmarco/libvirt,VenkatDatta/libvirt,VenkatDatta/libvirt,elmarco/libvirt,rlaager/libvirt,shugaoye/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,elmarco/libvirt,taget/libvirt,andreabolognani/libvirt,olafhering/libvirt,datto/libvirt,libvirt/libvirt,jardasgit/libvirt,zippy2/libvirt,zippy2/libvirt,zhlcindy/libvirt-1.1.4-maintain,nertpinx/libvirt,zhlcindy/libvirt-1.1.4-maintain,eskultety/libvirt,cbosdo/libvirt,agx/libvirt,nertpinx/libvirt,shugaoye/libvirt,shugaoye/libvirt,crobinso/libvirt,olafhering/libvirt