guest_state: Use references in ThreadState accessors

This brings these functions in line with the Google C++ Style Guide.

Bug: 288617924
Test: berberis_host_tests
Change-Id: I81c9f402672fdf418b3ff30618bdad719a527fa2
Merged-In: I81c9f402672fdf418b3ff30618bdad719a527fa2
11 files changed