Nested p2m: implement "flush" as a first-class action

Nested p2m: implement "flush" as a first-class action
rather than using the teardown and init functions.
This makes the locking clearer and avoids an expensive scan of all
pfns that's only needed for non-nested p2ms.  It also moves the
tlb flush into the proper place in the flush logic, avoiding a
possible race against other CPUs.

Signed-off-by: Tim Deegan <Tim.Deegan@citrix.com>
Acked-by: Christoph Egger <Christoph.Egger@amd.com>