CA-139258: Fix functional tests on iSCSI SR

Raise an exception from within xencert when iscsiadm discovery fails and
abort any further tests.

Signed-off-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>
Reviewed-by: Mate Lakat <mate.lakat@citrix.com>

GitHub: closes #18 on xenserver/xencert