qemu: snapshot: Skip 'transaction' command when no disks are selected
qemu: snapshot: Skip 'transaction' command when no disks are selected

When doing a memory-only snapshot libvirt would still issue the
'transaction' command without any disk. Skip it if it isn't necessary.

datto/libvirt,VenkatDatta/libvirt,VenkatDatta/libvirt,zippy2/libvirt,andreabolognani/libvirt,rlaager/libvirt,crobinso/libvirt,crobinso/libvirt,nertpinx/libvirt,rlaager/libvirt,datto/libvirt,andreabolognani/libvirt,taget/libvirt,eskultety/libvirt,taget/libvirt,rlaager/libvirt,andreabolognani/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,datto/libvirt,jardasgit/libvirt,olafhering/libvirt,libvirt/libvirt,jfehlig/libvirt,nertpinx/libvirt,crobinso/libvirt,jardasgit/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,eskultety/libvirt,VenkatDatta/libvirt,crobinso/libvirt,nertpinx/libvirt,rlaager/libvirt,datto/libvirt,eskultety/libvirt,rlaager/libvirt,fabianfreyer/libvirt,eskultety/libvirt,nertpinx/libvirt,VenkatDatta/libvirt,taget/libvirt,datto/libvirt,taget/libvirt,andreabolognani/libvirt,taget/libvirt,VenkatDatta/libvirt,fabianfreyer/libvirt,eskultety/libvirt,jardasgit/libvirt,nertpinx/libvirt,zippy2/libvirt,olafhering/libvirt,zippy2/libvirt,jardasgit/libvirt,libvirt/libvirt,zippy2/libvirt,olafhering/libvirt,jfehlig/libvirt,libvirt/libvirt,jardasgit/libvirt,libvirt/libvirt,jfehlig/libvirt,olafhering/libvirt