p2m: Allow non-leaf entries to be replaced by leaf entries

p2m: Allow non-leaf entries to be replaced by leaf entries

Allow l2 and l3 p2m tables to be replaced with 2MB and 1GB pages
respectively, freeing the p2m table page properly.  This allows, for example,
a sequence of 512 singleton zero pages to be replaced with a superpage
populate-on-demand entry.

Changes:
* Add a p2m_free_ptp() corresponding to p2m_alloc_ptp(), which will
handle everything related to the freeing properly.
* Add p2m_free_entry(), based on ept_free_entry(), which will free
intermediate tables recursively.
* For both ept and p2m, when replacing non-leaf entries with leaf
entries, keep old entry and call *_free_entry() after new entry
has been written and proper flushes have been done.

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