This patch introduces phys memory client for Xen.

Only sync dirty_bitmap and set_memory are actually implemented.
migration_log will stay empty for the moment.

Xen can only log one range for bit change, so only the range in the
first call will be synced.
Signed-off-by: NAnthony PERARD <anthony.perard@citrix.com>
Signed-off-by: NAlexander Graf <agraf@suse.de>