qemu-img: Fix qemu-img convert -obacking_file

The old -B option caused a backing file to be used for the converted image and
to avoid copying clusters from the old backing file. When replaced with
-obacking_file, qemu-img convert does assign the backing file to the new image,
but it doesn't realize that it should avoid copying clusters from the backing
file.

This patch checks the -o options for a backing_file and applies the same logic
as for -B in this case.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
