ANDROID: KVM: arm64: Add ptdump support for host stage-2

Register a new debugfs file to allow userspace tools to introspect the
host stage-2 pagetable layout. Hook the registration before the host
depriviledges to allow ptdump to read the number of pages reserved for
the stage-2 pagetables.

Bug: 260238318
Bug: 278749606
Change-Id: I1361c23e8911362bb03910004f241ca730a773b7
Signed-off-by: Sebastian Ene <sebastianene@google.com>