mem: Change the multilevel page table to inherit from FuncPageTable.

mem: Change the multilevel page table to inherit from FuncPageTable.

KVM looks up translations using the image of the page table in the
guest's memory, but we don't have to. By maintaining that image in
addition to rather than instead of maintaining an abstract copy makes
our lookups faster, and ironically avoids duplicate implementation.

Change-Id: I9ff4cae6f7cf4027c3738b75f74eae50dde2fda1
Reviewed-on: https://gem5-review.googlesource.com/7341
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>
Maintainer: Gabe Black <gabeblack@google.com>