hvmloader: Move option ROM loading into a separate optionnal file

hvmloader: Move option ROM loading into a separate optionnal file

Make load_rom field in struct bios_config an optionnal callback rather
than a boolean value. It allow BIOS specific code to implement it's
own option ROM loading methods.

Facilities to scan PCI devices, extract an deploy ROMs are moved into
a separate file that can be compiled optionnaly.

Signed-off-by: Julian Pidancet <julian.pidancet@gmail.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>