Remove old-style exception raising (pep3109)

The only exception is when raising with a traceback, for which
there is not a new-style equivalent both Python 2.x.

Change-Id: I1b669cb172eda12b90afb5f124e7935b03f668b4
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/10948
Reviewed-by: Zhou Zheng Sheng <zhshzhou@linux.vnet.ibm.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>