Xen: Use VGA Hooks to make VGA passthrough work on certain devices

Xen: Use VGA Hooks to make VGA passthrough work on certain devices

The Intel gfx VGA option ROM on certain platforms requires the 155f50 BIOS
function to be implemented (even if it does nothing), to work properly.

v2: Ignore the case where Option ROMs are pre-deployed.
    Make vgahook_setup independent of wether the CB* variables are set.
    VGA hooks can be enabled on non-coreboot and non-Xen configurations.

Signed-off-by: Julian Pidancet <julian.pidancet@gmail.com>