build: fix build with older libxml2

build: fix build with older libxml2

On RHEL 5, with libxml2-2.6.26, the build failed with:
virsh.c: In function 'vshNodeIsSuperset':
virsh.c:11951: warning: implicit declaration of function 'xmlChildElementCount'

(or if warnings aren't errors, a link failure later on).

* src/util/xml.h (virXMLChildElementCount): New prototype.
* src/util/xml.c (virXMLChildElementCount): New function.
* src/libvirt_private.syms (xml.h): Export it.
* tools/virsh.c (vshNodeIsSuperset): Use it.