Add support for use as Xen HVM BIOS.

Add support for use as Xen HVM BIOS.

SeaBIOS is called by Xen's hvmloader which does the basic platform
setup (PCI, APIC, etc) and provides the various BIOS tables. Therefore
avoid re-doing that setup and copy out the tables as necessary. A
simple data structure is defined to pass the addresses of these
tables.

This patch also establishes the basic infrastructure to make
hypercalls, although it currently only uses it to query the hypervisor
version.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>