bug fix: Import missing modules

cherrypy, sys, and logging are being used in Model implementation while
checking for default pool.
Import those modules to avoid errors.

Signed-off-by: Aline Manera <alinefm@br.ibm.com>
bug fix: Fix typo while logging information to cherrypy handlers

The cherrypy.log.error function has a parameter called 'severity' instead of
'serverity'. Fix it.
For reference: http://docs.cherrypy.org/dev/refman/_cplogging.html

Signed-off-by: Aline Manera <alinefm@br.ibm.com>
bug fix: Fix log error message when kimchi can not create default pool

The error message had a formatting mask (%s) but it was not being filled up.
Fix it.

Signed-off-by: Aline Manera <alinefm@br.ibm.com>
Create default network if it does not exist

The default network must exist and be active in order to Kimchi manage virtual
machines without problems.
So verify if the default network exists and if not create and active it.

Signed-off-by: Aline Manera <alinefm@br.ibm.com>