pvmdice: Initial pvmdice implementation

pvmdice is an implementation of hwbcc for use in protected vms where
a dice handover is provided to the guest OS. Here, we implement
initialization of PvmDice and the get_bcc function.

Bug: 369146791
Test: trusty/vendor/google/aosp/scripts/build.py qemu-generic-arm64-test-debug --test="boot-test:com.android.trusty.rust.pvmdice.test"

Change-Id: Ie5e87fdf518cfa025d9a20f5548eacbab87aab7b