x86/mm: Allocate log-dirty bitmaps from shadow/HAP memory.

x86/mm: Allocate log-dirty bitmaps from shadow/HAP memory.

Move the p2m alloc and free functions back into the per-domain paging
assistance structure and allow them to be called from the log-dirty
code.  This makes it less likely that log-dirty code will run out of
memory populating the log-dirty bitmap.

Signed-off-by: Tim Deegan <Tim.Deegan@citrix.com>