Modify naming of queries into the p2m

Modify naming of queries into the p2m

Callers of lookups into the p2m code are now variants of get_gfn. All
callers need to call put_gfn. The code behind it is a no-op at the
moment, but will change to proper locking in a later patch.

This patch does not change functionality. Only naming, and adds
put_gfn's.

set_p2m_entry retains its name because it is always called with
p2m_lock held.

This patch is humongous, unfortunately, given the dozens of call sites
involved.

After this patch, anyone using old style gfn_to_mfn will not succeed
in compiling their code. This is on purpose: adapt to the new API.

Signed-off-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>
Acked-by: Tim Deegan <tim@xen.org>
Committed-by: Keir Fraser <keir@xen.org>