xenFormatXLDisk: Don't leak @target

xenFormatXLDisk: Don't leak @target

==11260== 1,006 bytes in 1 blocks are definitely lost in loss record 106 of 111
==11260==    at 0x4C2AE5F: malloc (vg_replace_malloc.c:297)
==11260==    by 0x4C2BDFF: realloc (vg_replace_malloc.c:693)
==11260==    by 0x4EA430B: virReallocN (viralloc.c:245)
==11260==    by 0x4EA7C52: virBufferGrow (virbuffer.c:130)
==11260==    by 0x4EA7D28: virBufferAdd (virbuffer.c:165)
==11260==    by 0x4EA8E10: virBufferStrcat (virbuffer.c:718)
==11260==    by 0x42D263: xenFormatXLDiskSrcNet (xen_xl.c:960)
==11260==    by 0x42D4EB: xenFormatXLDiskSrc (xen_xl.c:1015)
==11260==    by 0x42D870: xenFormatXLDisk (xen_xl.c:1101)
==11260==    by 0x42DA89: xenFormatXLDomainDisks (xen_xl.c:1148)
==11260==    by 0x42EAF8: xenFormatXL (xen_xl.c:1558)
==11260==    by 0x40E85F: testCompareParseXML (xlconfigtest.c:105)

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>