run-op-bvt: use libXML to merge BVT XML files
run-op-bvt: use libXML to merge BVT XML files

The previous implementation did a bunch of nasty things with strings.
This is (obviously) not exactly bulletproof when it comes to dealing
with XML documents.

However - there is *excellent* news: we can use libXML (already a
dependency) to do exactly the same thing.

Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.vnet.ibm.com>

open-power/op-test-framework,open-power/op-test-framework,open-power/op-test-framework