python: Add XC binding for Xen build ID

python: Add XC binding for Xen build ID

Extend the list of xc() object methods with additional one to display
Xen's buildid. The implementation follows the libxl implementation
(e.g. max buildid size assumption being XC_PAGE_SIZE minus
sizeof(buildid->len)).

Signed-off-by: Pawel Wieczorkiewicz <wipawel@amazon.de>
Reviewed-by: Martin Mazein <amazein@amazon.de>
Reviewed-by: Andra-Irina Paraschiv <andraprs@amazon.com>
Reviewed-by: Norbert Manthey <nmanthey@amazon.de>
Acked-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>