dumpStorageTable: fixed a unhandled exception when poollist is empty

when poollist is empty then dumpStorageTable was failing with
'IndexError: list index out of range', Also crash dump was reported
in /var/log/messages

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=961816
Change-Id: I98b6bb29720cb21627dc8f1e23dd90126c3bf0b4
Signed-off-by: Aravinda VK <avishwan@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/14886
Reviewed-by: Dan Kenigsberg <danken@redhat.com>