fw_cfg: rom loader tweaks.

Changes:
 - make dir argument mandatory, we allways have one anyway
   (vgaroms or genroms).
 - check for duplicates, skip loading if found.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
