ANDROID: KVM: arm64: Have different callbacks for PTE manipulation

Move the host specific code for PTE reference counting out of the
pagetable code and define a new structure that wraps all the PTE
manipulation callbacks. This structure will be passed during the
pagetable code initialization and it allows to register different
callback for [guest|host].

Bug: 222044487
Signed-off-by: Sebastian Ene <sebastianene@google.com>
Change-Id: I116e8322935762df2f2be6e8d51a3f0c140b3d36
Signed-off-by: Quentin Perret <qperret@google.com>