Fix closing of /var/log/xend.log fd

Fix closing of /var/log/xend.log fd
Fix the problem where /var/log/xend.log was getting closed and then
/var/log/xend-debug was getting all the spam about the fd being closed
in the logging stuff.

Basically what was happening is that the main thread wasn't sticking
around and during its exit, the atexit handlers of the python logging
stuff got called closing the log fd.

Patch makes it so that we instead wait on the real server threads to
exit before shutting things down.

Signed-off-by: Jeremy Katz <katzj@redhat.com>
Signed-off-by: Christian Limpach <Christian.Limpach@cl.cam.ac.uk>