qemu: blockcopy: Refactor logic checking the target storage file
qemu: blockcopy: Refactor logic checking the target storage file

Use virStorageSource accessors to check the file and call
virStorageFileAccess before even attempting to stat the target. This
will be helpful once we try to add network destinations for block copy,
since there will be no need to stat them.

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