It works much better when parse_option_number actually
returns the number parsed ...

Common breakage resulting from this bug is that
'qemu -hda foo.img -cdrom bar.iso' stops working
(cdrom isn't there).

Cc: Avi Kivity <avi@redhat.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>