Fix recursive lock p2m lock acquisition in POD code

Fix recursive lock p2m lock acquisition in POD code

The POD code can take the p2m lock from inside a lookup.  This causes
a crash if anyone calls gfn_to_mfn* with the p2m lock held, which is
quite a few places.  Make the POD code understand that it may be
called with the lock held, and DTRT about talking or releasing it.

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