blockVolume: Fix NameError when validating volume parameters

Commit 5cd8c83ba62 was liberally verified, leaving undefined name. Fixed
now thanks to pyflakes and make check.

Change-Id: Ib0e7333dafd6adfec37ae880f51aa0a98978641b
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28426
Reviewed-by: Dan Kenigsberg <danken@redhat.com>