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)
add repr for event objects

When an event object gets dumped in a stack trace, because there
is no __repr__ of it, we don't get any details about what it was.
Add a __repr__ to all the event objects so that we can get this
information on a fail.

Closes-Bug: #1357063

Change-Id: I9a62a4348871b748c53b7354ad1b821fe987127d
(cherry picked from commit 8c98b601f2db1f078d5f42ab94043d9939608f73)
Merge "make lifecycle event logs more clear" into stable/icehouse
Merge "add repr for event objects" into stable/icehouse