util: add missing string->integer conversion functions

util: add missing string->integer conversion functions

It was awkward having only int conversion in the virStrToLong family,
but only long conversion in the virXPath family.  Make both families
support both types.

* src/util/util.h (virStrToLong_l, virStrToLong_ul): New
prototypes.
* src/util/xml.h (virXPathInt, virXPathUInt): Likewise.
* src/util/util.c (virStrToLong_l, virStrToLong_ul): New
functions.
* src/util/xml.c (virXPathInt, virXPathUInt): Likewise.
* src/libvirt_private.syms (util.h, xml.h): Export them.