roms: rework rom loading via fw

This patch changes the way rom loading via fw_cfg is handled.
Instead of having pc_init1() call a function which passed all
roms to the firmware config we simply pass a pointer to fw_cfg
to the rom loader.

Advantage: loading roms via firmware works also for devices which
are initialized after pc_init1(), i.e. everyting added via -device.

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