Fix memory leak in virDomainDefParseXML()
    
    This patch fixes the memory leaks found while running qemuxml2argvtest
    
    ==8260== 3 bytes in 1 blocks are definitely lost in loss record 1 of
    129
    ==8260==    at 0x4A0887C: malloc (vg_replace_malloc.c:270)
    ==8260==    by 0x341F485E21: strdup (strdup.c:42)
    ==8260==    by 0x4CADCFF: virStrdup (virstring.c:554)
    ==8260==    by 0x4CBB839: virXPathString (virxml.c:90)
    ==8260==    by 0x4CE753A: virDomainDefParseXML (domain_conf.c:11478)
    ==8260==    by 0x4CEB4FE: virDomainDefParseNode (domain_conf.c:12742)
    ==8260==    by 0x4CEB675: virDomainDefParse (domain_conf.c:12684)
    ==8260==    by 0x425958: testCompareXMLToArgvHelper (qemuxml2argvtest.c:107)
    ==8260==    by 0x427111: virtTestRun (testutils.c:138)
    ==8260==    by 0x41D3FE: mymain (qemuxml2argvtest.c:452)
    ==8260==    by 0x4277B2: virtTestMain (testutils.c:593)
    ==8260==    by 0x341F421A04: (below main) (libc-start.c:225)
    ==8260==
    ==8260== 4 bytes in 1 blocks are definitely lost in loss record 5 of
    129
    ==8260==    at 0x4A0887C: malloc (vg_replace_malloc.c:270)
    ==8260==    by 0x341F485E21: strdup (strdup.c:42)
    ==8260==    by 0x4CADCFF: virStrdup (virstring.c:554)
    ==8260==    by 0x4CBB839: virXPathString (virxml.c:90)
    ==8260==    by 0x4CE753A: virDomainDefParseXML (domain_conf.c:11478)
    ==8260==    by 0x4CEB4FE: virDomainDefParseNode (domain_conf.c:12742)
    ==8260==    by 0x4CEB675: virDomainDefParse (domain_conf.c:12684)
    ==8260==    by 0x425958: testCompareXMLToArgvHelper (qemuxml2argvtest.c:107)
    ==8260==    by 0x427111: virtTestRun (testutils.c:138)
    ==8260==    by 0x41D39A: mymain (qemuxml2argvtest.c:451)
    ==8260==    by 0x4277B2: virtTestMain (testutils.c:593)
    ==8260==    by 0x341F421A04: (below main) (libc-start.c:225)
    ==8260==