Resolve valgrind error in virNetDevVlanParse()

Resolve valgrind error in virNetDevVlanParse()

Commit '861d4056' introduced the following:

TEST: networkxml2xmltest
      ..................                       18  OK
==25504== 7 bytes in 1 blocks are definitely lost in loss record 5 of 23
==25504==    at 0x4A0887C: malloc (vg_replace_malloc.c:270)
==25504==    by 0x37C1085D71: strdup (strdup.c:42)
==25504==    by 0x4CB835F: virStrdup (virstring.c:546)
==25504==    by 0x4CC5179: virXPathString (virxml.c:90)
==25504==    by 0x4CC75C2: virNetDevVlanParse (netdev_vlan_conf.c:78)
==25504==    by 0x4CF928A: virNetworkPortGroupParseXML (network_conf.c:1555)
==25504==    by 0x4CFE385: virNetworkDefParseXML (network_conf.c:2049)
==25504==    by 0x4D0113B: virNetworkDefParseNode (network_conf.c:2273)
==25504==    by 0x4D01254: virNetworkDefParse (network_conf.c:2234)
==25504==    by 0x401E80: testCompareXMLToXMLHelper (networkxml2xmltest.c:32)
==25504==    by 0x402D4F: virtTestRun (testutils.c:158)
==25504==    by 0x401CE9: mymain (networkxml2xmltest.c:110)
==25504==
PASS: networkxml2xmltest

Also changed the label from error to cleanup and adjusted code since it's
all one exit path