qemu: blockcopy: Split out checking of the target image file
qemu: blockcopy: Split out checking of the target image file

Move the code into a separate function so that the flow of creating the
copy is more obvious and split into logical pieces.

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