xenpaging: break endless loop during inital page-out with large pagefiles

xenpaging: break endless loop during inital page-out with large pagefiles

To allow the starting for xenpaging right after 'xm start XYZ', I
specified a pagefile size equal to the guest memory size in the hope
to catch more errors where the paged-out state of a p2mt is not
checked.

While doing that, xenpaging got into an endless loop because some
pages cant be paged out right away. Now the policy reports an error if
the gfn number wraps.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Already-Acked-by: Patrick Colp <pjcolp@cs.ubc.ca>
Already-Acked-by: Keir Fraser <keir.fraser@citrix.com>