virsh-snapshot: Refactor cmdSnapshotDumpXML
virsh-snapshot: Refactor cmdSnapshotDumpXML

This patch simplifies error paths and switches to use
vshCommandOptStringReq for argument retrieval in cmdSnapshotDumpXML

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