Improve xattr test failure
Improve xattr test failure

TransportProtocols/XattrTest.MB_23882_VirtualXattrs has been
seen to fail as a vattr wasn't present, we need the key of the
vattr to assist any further debug.

Change-Id: Ie296cc18d12d3548bf0359b603e698757e85f750
Reviewed-on: http://review.couchbase.org/99556
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine