qemu-img: Suppress unhelpful extra errors in convert, resize

add_old_style_options() for img_convert() and img_resize() use
qemu_opt_set(), which reports errors with qerror_report_err().  Its
error messages aren't helpful here, the caller reports one that
actually makes sense.  Reproducer:

    $ qemu-img convert -B raw in.img out.img
    qemu-img: Invalid parameter 'backing_file'
    qemu-img: Backing file not supported for file format 'raw'

Switch to qemu_opt_set_err() to get rid of the unwanted messages.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
