LP1937294: Fix Vandelay Item Import for Pg11+

LP1937294: Fix Vandelay Item Import for Pg11+

Modify the vandelay.ingest_items() function to use proper XPath format
for relative node searching.  This change was necessitated by a fix in
Pg 11's Xpath code:

 * Correctly handle relative path expressions in xmltable(), xpath(), and
   other XML-handling functions

The lp1379815_vl_import_item_stat_cats.pg PgTap regression test fails
on PostgreSQL 11+ if this patch is not applied and succeeds if it
does.  It still passes on PostgreSQL 10.

Signed-off-by: Jason Stephenson <jason@sigio.com>