Our seclabel parsing was repeatedly assigning malloc'd data into a
temporary variable, without first freeing the previous use.  Among
other leaks flagged by valgrind:

==9312== 8 bytes in 1 blocks are definitely lost in loss record 88 of 821
==9312==    at 0x4A0645D: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==9312==    by 0x8C40369: strdup (strdup.c:42)
==9312==    by 0x50EA799: virStrdup (virstring.c:676)
==9312==    by 0x50FAEB9: virXPathString (virxml.c:90)
==9312==    by 0x50FAF1E: virXPathStringLimit (virxml.c:112)
==9312==    by 0x510F516: virSecurityLabelDefParseXML (domain_conf.c:4571)
==9312==    by 0x510FB20: virSecurityLabelDefsParseXML (domain_conf.c:4720)

While it was multiple problems, it looks like commit da78351b (thankfully
unreleased) was to blame for all of them.

* src/conf/domain_conf.c (virSecurityLabelDefParseXML): Plug leaks
detected by valgrind.
Signed-off-by: NEric Blake <eblake@redhat.com>