policy_choose_victim() is one of the bottlenecks in xenpaging. It is called
alot to find free bits in the fragmented bitmaps.

Reduce turnaround time by skipping longs with all bits set.
Adjust wrap detection in loop.

v2:
 - fix copy&paste error, bitmap was tested twice

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Committed-by: Ian Jackson <ian.jackson.citrix.com>