xen: Introduce xen_modified_memory.

This function is to be used during live migration. Every write access to the
guest memory should call this funcion so the Xen tools knows which pages are
dirty.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
diff --git a/xen-stub.c b/xen-stub.c
index 5e66ba8..9214392 100644
--- a/xen-stub.c
+++ b/xen-stub.c
@@ -59,3 +59,7 @@
 void qmp_xen_set_global_dirty_log(bool enable, Error **errp)
 {
 }
+
+void xen_modified_memory(ram_addr_t start, ram_addr_t length)
+{
+}