xen: update xen_add_to_physmap interface

Update struct xen_add_to_physmap to be in sync with Xen's version of the
structure.
The size field was introduced by:

changeset:   24164:707d27fe03e7
user:        Jean Guyader <jean.guyader@eu.citrix.com>
date:        Fri Nov 18 13:42:08 2011 +0000
summary:     mm: New XENMEM space, XENMAPSPACE_gmfn_range

According to the comment:

"This new field .size is located in the 16 bits padding between .domid
and .space in struct xen_add_to_physmap to stay compatible with older
versions."

Changes in v2:

- remove erroneous comment in the commit message.
Signed-off-by: NStefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: NKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>

include/xen/interface/memory.h

@@ -163,6 +163,9 @@ struct xen_add_to_physmap {
     /* Which domain to change the mapping for. */
     domid_t domid;
 
+    /* Number of pages to go through for gmfn_range */
+    uint16_t    size;
+
     /* Source mapping space. */
 #define XENMAPSPACE_shared_info 0 /* shared info page */
 #define XENMAPSPACE_grant_table 1 /* grant table page */