passthrough: basic graphics passthrough support

passthrough: basic graphics passthrough support

basic gfx passthrough support:
  - add a vga type for gfx passthrough
  - retrieve VGA bios from host 0xC0000, then load it to guest 0xC0000
  - register/unregister legacy VGA I/O ports and MMIOs for passthroughed gfx

Signed-off-by: Ben Lin <ben.y.lin@intel.com>
Signed-off-by: Weidong Han <weidong.han@intel.com>
Acked-by: Jean Guyader <jean.guyader@critix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>