Add Xen PV block device support to SeaBIOS

by Daniel Castro for Xen.org

Currently Xen is using for HVM Virtual machines a version of BIOS called rombios, ROMBIOS is functional but maintenance is difficult, it is a mixture of C and inline assembly. Xen wants to migrate to SeaBIOS, which happens to also be the qemu upstream and KVM BIOS of choice, that means that the source code is more maintainable. Then a modification to int0x13 will be done to use the PV Drivers via XenStore instead of the emulated device used in qemu.