mshv: handle gpa intercepts for arm64

mshv: handle gpa intercepts for arm64

The mshv driver now uses movable pages for guests. For arm64 guests
to be functional, handle gpa intercepts for arm64 too (the current
code implements handling only for x86).

Move some arch-agnostic functions out of #ifdefs so that they can be
re-used.

Fixes: b9a66cd5ccbb ("mshv: Add support for movable memory regions")
Signed-off-by: Anirudh Rayabharam (Microsoft) <anirudh@anirudhrb.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>