xenpaging: handle temporary out-of-memory conditions during page-in

xenpaging: handle temporary out-of-memory conditions during page-in

p2m_mem_paging_prep() should return -ENOMEM if a new page could not be
allocated. This can be handled in xenpaging to retry the
page-in. Right now such condition would stall the guest because the
requested page will not come back, xenpaging simply exits. So
xenpaging could very well retry the allocation forever to rescue the
guest.

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