hvmloader: refactor BIOS info setup

hvmloader: refactor BIOS info setup

Currently we have ->bios_high_setup, which is called relatively early
and returns a cookie which is passed to ->bios_info_setup which runs
towards the end and creates the BIOS info, incorporating the cookie
which (in the case of ROMBIOS) happens to be the BIOS's high load
address . This is rather ROMBIOS specific.

Refactor to have ->bios_info_setup which is called early and prepares
the bios_info, ->bios_relocate which does any necessary relocation
(updating the BIOS info as necessary) and ->bios_info_finish which
finalises the info (e.g.  by calculating the checksum).

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