conf: Plug memory leaks on virDomainDiskDefParseXML

conf: Plug memory leaks on virDomainDiskDefParseXML

Detected by valgrind. Leaks are introduced in commit b22eaa7.

* src/conf/domain_conf.c (virDomainDiskDefParseXML): fix memory leaks.

How to reproduce?

% make && make -C tests check TESTS=qemuxml2argvtest
% cd tests && valgrind -v --leak-check=full ./qemuxml2argvtest

actual result:

==2143== 12 bytes in 2 blocks are definitely lost in loss record 74 of 179
==2143==    at 0x4A05FDE: malloc (vg_replace_malloc.c:236)
==2143==    by 0x39D90A67DD: xmlStrndup (xmlstring.c:45)
==2143==    by 0x4F5EC0: virDomainDiskDefParseXML (domain_conf.c:3438)
==2143==    by 0x502F00: virDomainDefParseXML (domain_conf.c:8304)
==2143==    by 0x505FE3: virDomainDefParseNode (domain_conf.c:9080)
==2143==    by 0x5069AE: virDomainDefParse (domain_conf.c:9030)
==2143==    by 0x41CBF4: testCompareXMLToArgvHelper (qemuxml2argvtest.c:105)
==2143==    by 0x41E5DD: virtTestRun (testutils.c:145)
==2143==    by 0x416FA3: mymain (qemuxml2argvtest.c:399)
==2143==    by 0x41DCB7: virtTestMain (testutils.c:700)
==2143==    by 0x39CF01ECDC: (below main) (libc-start.c:226)

Signed-off-by: Alex Jia <ajia@redhat.com>