maint: prohibit most uses of xmlGetProp

maint: prohibit most uses of xmlGetProp

Making this change makes it easier to spot the memory leaks
that will be fixed in the next patch.

* cfg.mk (sc_prohibit_xmlGetProp): New rule.
* .x-sc_prohibit_xmlGetProp: New exception.
* Makefile.am (EXTRA_DIST): Ship exception file.
* tools/virsh.c (cmdDetachInterface, cmdDetachDisk): Adjust
offenders.
* src/conf/storage_conf.c (virStoragePoolDefParseSource):
Likewise.
* src/conf/network_conf.c (virNetworkDHCPRangeDefParseXML)
(virNetworkIPParseXML): Likewise.