virXPathNodeSet() could return -1 when doing an evaluation failure
due to xmlXPathEval() from libxml2 behaviour.
* src/util/xml.c: make sure we always return 0 unless the returned
XPath type is of the wrong type (meaning the query passed didn't
evaluate to a node set and code must be fixed)
* src/util/xml.c: The virXPath... function take extra care to preserve
the XPath context node (ctxt->node) but in the case of virXPathString
and virXPathBoolean they forgot to do this on the error path. This
patch fixes this and move all ctxt->node = relnode instuctions just
after the xmlXPathEval() to make sure this doesn't happen if this code
is modified.
