x86/P2M: introduce p2m_{add,remove}_page()

x86/P2M: introduce p2m_{add,remove}_page()

Rename guest_physmap_add_entry() to p2m_add_page(); make
guest_physmap_remove_page() a trivial wrapper around p2m_remove_page().
This way callers can use suitable pairs of functions (previously
violated by hvm/grant_table.c).

In HVM-specific code further avoid going through the guest_physmap_*()
layer, and instead use the two new/renamed functions directly.

Ultimately the goal is to have guest_physmap_...() functions cover all
types of guests, but p2m_...() dealing only with translated ones.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Paul Durrant <paul@xen.org>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>