conf: tighten up XML integer parsing

conf: tighten up XML integer parsing

https://bugzilla.redhat.com/show_bug.cgi?id=617711 reported that
even with my recent patched to allow <memory unit='G'>1</memory>,
people can still get away with trying <memory>1G</memory> and
silently get <memory unit='KiB'>1</memory> instead.  While
virt-xml-validate catches the error, our C parser did not.

Not to mention that it's always fun to fix bugs while reducing
lines of code.  :)

* src/conf/domain_conf.c (virDomainParseMemory): Check for parse error.
(virDomainDefParseXML): Avoid strtoll.
* src/conf/storage_conf.c (virStorageDefParsePerms): Likewise.
* src/util/xml.c (virXPathLongBase, virXPathULongBase)
(virXPathULongLong, virXPathLongLong): Likewise.