virsh-snapshot: Update snapshot commands to use vshCommandOptStringReq
virsh-snapshot: Update snapshot commands to use vshCommandOptStringReq

also avoids potential NULL pointer dereference:

$ virsh snapshot-current asdf ""
error: invalid snapshotname argument '(null)'

by removing the error message in favor of vshCommandOptStringReq

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