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