KVM: selftests: arm64: Fix pte encode/decode for PA bits > 48

The high bits [51:48] of a physical address should appear at [15:12] in
a 64K pte, not at [51:48] as was previously being programmed. Fix this
with new helper functions that do the conversion correctly. This also
sets us up nicely for adding LPA2 encodings in future.

Fixes: 7a6629ef ("kvm: selftests: add virt mem support for aarch64")

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>