clientIF: Log unhandled exception for new Thread

Adding a traceback log for unhandled exceptions, when openning a new thread, so
it will not die silently.  Since the log in clientIF instance is inaccessible
from the decorator, we use the default root logger.

Change-Id: I595902fd9d7b62879a4466d4f8deae55a0ed7580
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/22812
Reviewed-by: Dan Kenigsberg <danken@redhat.com>