vdsm: Add additional exception handling within vmChannels.py

At present any exception thrown by _epoll.poll(1) is not caught. This patch adds
additional exception handling to catch and log any exceptions thrown by the
thread.

Change-Id: Ie733ca7ccd2689f6041ee18bbe335f355a89ac55
Bug-Url: https://bugzilla.redhat.com/918541
Signed-off-by: Lee Yarwood <lyarwood@redhat.com>
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/13093
Reviewed-by: Jason Dillaman <jdillama@redhat.com>
Reviewed-by: Peter V. Saveliev <peet@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Gal Hammer <ghammer@redhat.com>
Tested-by: Jason Dillaman <jdillama@redhat.com>
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>