Fix memory leak in virDomainSnapshotDiskDefClear()

Fix memory leak in virDomainSnapshotDiskDefClear()

While running domainsnapshotxml2xmltest, it was found that valgrind pointed out
the following memory leak:

==32176== 42 (32 direct, 10 indirect) bytes in 1 blocks are definitely lost in loss record 42 of 66
==32176==    at 0x4A069EE: malloc (vg_replace_malloc.c:270)
==32176==    by 0x4A06B62: realloc (vg_replace_malloc.c:662)
==32176==    by 0x4C65A07: virReallocN (viralloc.c:243)
==32176==    by 0x4C65B2E: virExpandN (viralloc.c:292)
==32176==    by 0x4C65E30: virInsertElementsN (viralloc.c:434)
==32176==    by 0x4CD71F3: virDomainDiskSourceDefParse (domain_conf.c:5078)
==32176==    by 0x4CF6EF4: virDomainSnapshotDefParseNode (snapshot_conf.c:151)
==32176==    by 0x4CF7314: virDomainSnapshotDefParseString (snapshot_conf.c:410)
==32176==    by 0x41FB8D: testCompareXMLToXMLHelper (domainsnapshotxml2xmltest.c:100)
==32176==    by 0x420FD1: virtTestRun (testutils.c:199)
==32176==    by 0x41F859: mymain (domainsnapshotxml2xmltest.c:222)
==32176==    by 0x42174D: virtTestMain (testutils.c:782)
==32176==
... and one more.