volume: improve volume clone logging

This patch adds the traceback output in case of an exception,
removes the superfluous "Volume.clone:" prefixes and uses the
correct formatting.

Change-Id: Id3fb5f9cc74792c7bd94ccbc49cc39b00d464370
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26867
Reviewed-by: Dan Kenigsberg <danken@redhat.com>