This allows using the file.filename option instead of the string that
comes from -drive file=... and is passed around as a separate parameter.
The goal is to get rid of this parameter and use the options QDict more
consistently.

With this option you can access not only the top-level image, but
specify a filename for the backing file (currently only if no backing
file exists, but we'll allow overriding it later)

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