tools/hotplug: read /etc/default/xencommons if appropriate

tools/hotplug: read /etc/default/xencommons if appropriate

Since 22187:c41252a55a0a we have been installing our example
xencommons settings file in either /etc/sysconfig or /etc/default,
depending on whether /etc/sysconfig exists.

However I omitted to add the code to /etc/init.d/xencommons to
actually read either version of the file, although every other init
script seems to have it.

An effect of this misplaced/unread file is that the automatic tests
don't cause xenconsoled to collect serial logs, because the tester
edits whichever file actually exists.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>