prep: Fix offset of BIOS MemoryRegion

prep: Fix offset of BIOS MemoryRegion

Since 0c90c52fab5ea92d7f12b29bfe26a7cd75d9efcb (ppc_prep: convert to memory
API) OHW was "Trying to execute code outside RAM or ROM at 0xfff00700".

The BIOS MemoryRegion is created with a fixed size of 1 MiB.
Ensure that the full size can be accessed since the exception
vectors are located at 0xfff00000 and the BIOS may want to use them.

It thereby no longer depends on the actual BIOS binary size.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Cc: Avi Kivity <avi@redhat.com>
Cc: Alexander Graf <agraf@suse.de>