virDomainDefParseXML: Free @tmp when parsing genid
    
    We need to free return value of virXPathString().
    
    ==12962== 37 bytes in 1 blocks are definitely lost in loss record 156 of 331
    ==12962==    at 0x4C2AF0F: malloc (vg_replace_malloc.c:299)
    ==12962==    by 0x91E8439: strdup (in /lib64/libc-2.25.so)
    ==12962==    by 0x5DBD551: virStrdup (virstring.c:977)
    ==12962==    by 0x5DD3E5E: virXPathString (virxml.c:84)
    ==12962==    by 0x5E178AB: virDomainDefParseXML (domain_conf.c:19110)
    ==12962==    by 0x5E1E985: virDomainDefParseNode (domain_conf.c:20885)
    ==12962==    by 0x5E1E7CB: virDomainDefParse (domain_conf.c:20827)
    ==12962==    by 0x5E1E871: virDomainDefParseFile (domain_conf.c:20853)
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>