virsh: Refactor argument handling in cmdBlockCopy
virsh: Refactor argument handling in cmdBlockCopy

Put all argument parsing together and refactor the argument checking
code.

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