Prevent Upstart from restarting guest in Restart tests

Stop the mysql service explicitly during the blackbox restart
tests, so that upstart does not try to re-spawn the mysql process
and cause test failures. This is needed for the blackbox tests to
successfully run.

Fixes bug: 1110895

Change-Id: I44cfeac3350913af5b6cb05b8b3373ab84c448f7
Merge "Prevent Upstart from restarting guest in Restart tests"