virsh-snapshot: Refactor virsh snapshot-list
virsh-snapshot: Refactor virsh snapshot-list

Simplify error handling and mutually exclusive option checking.

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