PoD,hap: Fix logdirty mode when using hardware assisted paging

PoD,hap: Fix logdirty mode when using hardware assisted paging

When writing a writable p2m entry for a pfn, we need to mark the pfn
dirty to avoid corruption when doing live migration.

Marking the page dirty exposes another issue, where there are
excessive sweeps for zero pages if there's a mismatch between PoD
entries and cache entries.  Only sweep for zero pages if we actually
need more memory.

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