Export all symbols from xml.h for internal use

Some of the very useful calls for XML parsing provided by util/xml.[ch]
were not exported as private symbols. This patch fixes this.
Signed-off-by: NJiri Denemark <jdenemar@redhat.com>

src/libvirt_private.syms

@@ -543,3 +543,9 @@ virXPathNodeSet;
 virXPathString;
 virXMLPropString;
 virXPathStringLimit;
+virXPathBoolean;
+virXPathNumber;
+virXPathULong;
+virXPathULongLong;
+virXPathLongHex;
+virXPathULongHex;