xenpaging: track number of paged pages in struct domain

xenpaging: track number of paged pages in struct domain

The toolstack should know how many pages are paged-out at a given point
in time so it could make smarter decisions about how many pages should
be paged or ballooned.

Add a new member to xen_domctl_getdomaininfo and bump interface version.
Use the new member in xc_dominfo_t.
The SONAME of libxc should be changed if this patch gets applied.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Tim Deegan <tim@xen.org>
Committed-by: Tim Deegan <tim@xen.org>