LP:685809 - Occasional failure to start mysqd seen in buildbot

Very occasionally, we see the following failure in the "install" tests for
.deb package builds in Buildbot:

Setting up mariadb-server-5.1 (5.1.51-mariadb94) ...
 * Stopping MariaDB database server mysqld
   ...done.
101123 12:02:59 [Note] Plugin 'PBXT' is disabled.
101123 12:02:59 [Note] Plugin 'InnoDB' is disabled.
 * Starting MariaDB database server mysqld
   ...fail!
invoke-rc.d: initscript mysql, action "start" failed.
dpkg: error processing mariadb-server-5.1 (--configure):
 subprocess installed post-installation script returned error exit status 1

It occurs quite rarely, maybe around 1% of runs or less, so quite difficult to
reproduce. Seen in both 5.1 and 5.2.

To proceed further understanding the problem, it would be very useful to have
access to log files after a test run, in particular /var/log/daemon.log in
this case.

One way to do this would be to archive the virtual image after each build or
test run (the image is currently discarded). This requires some disk space,
but should be otherwise reasonable simple to implement.