util: xml: Reimplement virXPath(U)Int via virXPathEvalString

util: xml: Reimplement virXPath(U)Int via virXPathEvalString

Similarly to the refactor of virXPath(U)LongLong drop the ability to
convert from the internal double value forcing the use of the 'string()'
conversion.

In case of 32 bit integers there's no problem with overflows, but we can
implement the code identically to what we have in the other helpers.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>