make lifecycle event logs more clear

Instead of logging integers in a cryptic way, log the human
readable strings for VM event state transitions. Also use the
existing instance_uuid port of the logger instead of putting that
separately into our log message.

Partial-Bug: #1357063

Change-Id: I96c39ad4494aa06a8dd5bd942c669a16836a8b9c
(cherry picked from commit 79c8bee9be7b6cf4e9fd3365a162f93a2b014bb5)
Merge "make lifecycle event logs more clear" into stable/icehouse