virt: log.error(exc_info=True) -> log.exception()

This patch just replaces the occurrences of

log.error('something', exc_info=True)

with the recommended idiom

log.exception('something').

No changes in behaviour.

Change-Id: I675d4dda74e2eaff05f127836c821d1c7d0a2b53
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/34353
Reviewed-by: Dan Kenigsberg <danken@redhat.com>