block: Fix the use of protocols in backing files Backing filenames may contain a protocol. The code currently doesn't consider this case and produces filenames that embed "<protocol>:". Don't combine filenames if the backing filename contains a protocol. Based on an earlier patch by Anthony Liguori <aliguori@us.ibm.com>. Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>