xen: Add xc_domain_add_to_physmap to xen_interface.

This function will be used to support sync dirty bitmap.

This come with a check against every Xen release, and special
implementation for Xen version that doesn't have this specific call.

This function will not be usable with Xen 3.3 because the behavior is
different.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
