Extract a library for common CompOS things

A small refactoring as a preliminary step to creating composd.

Create a Rust library for useful things shared by various
CompOS-related binaries. This initially includes the code to start the
VM and connect to CompOS, as well as various useful constants.

As part of extracting the start VM code I migrated to using logging
directly rather than writing to stdout/stderr for greater reusability,
as suggested by Victor.

Bug: 186126194
Test: Get odsign to run compos_verify_key, still works
Change-Id: I57b7ebcdd1a6cb604b5d739b8a8e028fd59e7b90