Expose fw_cfg v2

Multiboot passes options to the option rom using the fw_cfg device.
Right now, that device is local to the bochs_bios_init function.

Let's change that and expose it, so everyone may put data in there.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
