VMware: fix regression attaching iscsi cinder volumes

VMware drivers fail to attach a iSCSI backed cinder volume with an
error 'unable to find iscsi targets' because the underlying
retrieval api was updated to a newer version but not it's usage in
volume_util (see commit e331287cdd1300878bfb9bbb3630889370e5fa72).
This is a regression over Grizzly release.

Fixes: bug #1193980

(cherry picked from commit e70211ff89785fed6a868eaab723847833590502)

Change-Id: I14fe62ad98fd9161df3ce6c06160b2759a9e38b2