KVM: Abstract/rework instance up checks

This patch abstract the check "is instance stopped" into a separate
function, and thus simplifies a couple of higher-level functions. It
also moves from manual read of the pidfile to use the (correct
abstraction of) _InstancePidAlive.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Olivier Tharan <olive@google.com>