block: Add driver-specific options for backing files

Options starting in "backing." are passed to the backing file now. If
you don't need to specify the filename for the backing file, you can add
it on the command line instead of in the image file:

$ qemu-nbd -t /tmp/test.img
$ qemu-img create -f qcow2 empty.qcow2 1G
$ qemu-system-x86_64 -drive file=empty.qcow2,backing.file.driver=nbd,\
    backing.file.host=localhost

Note that this doesn't override the backing filename from the image. If
the image has one, this will fail because NBD doesn't want the options
and a filename at the same time.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
