This patch introduces Xen specific call in piix_pci.

The specific part for Xen is in write_config, set_irq and get_pirq.
Signed-off-by: NAnthony PERARD <anthony.perard@citrix.com>
Signed-off-by: NStefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: NAlexander Graf <agraf@suse.de>
Signed-off-by: NAlexander Graf <agraf@suse.de>