xenpaging: notify policy only on resume

xenpaging: notify policy only on resume

If a page is requested more than once, the policy is also notified
more than once about the page-in. However, a page-in happens only
once. Any further resume will only unpause the other vcpu. The
multiple notify will put the page into the mru list multiple times and
it will unlock other already resumed pages too early. In the worst
case, a page that was just resumed can be evicted right away, causing
a deadlock in the guest.

Signed-off-by: Olaf Hering <olaf@aepfle.de>