virsh: Extract fallback handling in cmdVcpuinfo
virsh: Extract fallback handling in cmdVcpuinfo

Put it into a separate function so that more fallback handling can be
added without making a mess.

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