virDomainDiskBackingStoreParse: Don't leak @idx
    
    ==1277== 8 bytes in 4 blocks are definitely lost in loss record 39 of 131
    ==1277==    at 0x4C2AEDF: malloc (vg_replace_malloc.c:299)
    ==1277==    by 0x68BBBC8: xmlStrndup (in /usr/lib64/libxml2.so.2.9.4)
    ==1277==    by 0x53B1DC2: virXMLPropString (virxml.c:510)
    ==1277==    by 0x53D696A: virDomainDiskBackingStoreParse (domain_conf.c:8639)
    ==1277==    by 0x53DA684: virDomainDiskDefParseXML (domain_conf.c:9590)
    ==1277==    by 0x53F619F: virDomainDefParseXML (domain_conf.c:19233)
    ==1277==    by 0x53F96EE: virDomainDefParseNode (domain_conf.c:20083)
    ==1277==    by 0x53F9540: virDomainDefParse (domain_conf.c:20027)
    ==1277==    by 0x53F95E6: virDomainDefParseFile (domain_conf.c:20053)
    ==1277==    by 0x44D1D4: testCompareDomXML2XMLFiles (testutils.c:1265)
    ==1277==    by 0x42FC7C: testXML2XMLActive (qemuxml2xmltest.c:71)
    ==1277==    by 0x44AD20: virTestRun (testutils.c:180)
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>