PoD memory 5/9: emergency scan

PoD memory 5/9: emergency scan

Implement "emergency scan" for zero pages, to deal with start-of-day
page scrubbers.

If the cache is running out, scan through memory looking for "zero
pages" that we can reclaim for the cache.  This is necessary for
operating systems which have a start-of-day page scrubber which runs
before the balloon driver can balloon down to the target.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>