Replace VM.platform_{std_VGA,serial,localtime,clock_offset,enable_audio} with

Replace VM.platform_{std_VGA,serial,localtime,clock_offset,enable_audio} with
a single VM.platform String -> String Map.

Propogate this change through to XendDomainInfo and image so that the
backwards compatibility is handled by XendConfig.

Make PCI_bus a read-write field.

Added C bindings for these changes also.

Signed-off-by: Ewan Mellor <ewan@xensource.com>