virDomainNetDefClear: Free @persistent name

virDomainNetDefClear: Free @persistent name

The persistent alias name @persistent is allocated in
virDomainNetDefParseXML() but never freed.

==119642== 22 bytes in 2 blocks are definitely lost in loss record 178 of 671
==119642==    at 0x483579F: malloc (vg_replace_malloc.c:309)
==119642==    by 0x58F89F1: xmlStrndup (in /usr/lib64/libxml2.so.2.9.9)
==119642==    by 0x4BA3B74: virXMLPropString (virxml.c:520)
==119642==    by 0x4BDB0C5: virDomainNetDefParseXML (domain_conf.c:11876)
==119642==    by 0x4BF9EF4: virDomainDefParseXML (domain_conf.c:21196)
==119642==    by 0x4BFCD5B: virDomainDefParseNode (domain_conf.c:21943)
==119642==    by 0x4BFCC36: virDomainDefParse (domain_conf.c:21901)
==119642==    by 0x4BFCCCB: virDomainDefParseFile (domain_conf.c:21924)
==119642==    by 0x114A9D: testCompareXMLToArgv (qemuxml2argvtest.c:452)
==119642==    by 0x13894F: virTestRun (testutils.c:143)
==119642==    by 0x11F46E: mymain (qemuxml2argvtest.c:1316)
==119642==    by 0x13A60E: virTestMain (testutils.c:839

Fixes: fb0509d06ac57434c2edbd81ee63deb32a0e598a
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>