virsh: plug memory leak on cmdDomblklist

Detected by valgrind. Leak introduced in commit 88a993b1:

* tools/virsh.c: fix memory leak on cmdDomblklist.

* how to reproduce?
  % valgrind -v --leak-check=full virsh domblklist <domain name>

* actual valgrind result:

==6573== 1,836 bytes in 1 blocks are definitely lost in loss record 110 of 124
==6573==    at 0x4A05FDE: malloc (vg_replace_malloc.c:236)
==6573==    by 0x330D71497D: xdr_string (in /lib64/libc-2.12.so)
==6573==    by 0x4D26CED: xdr_remote_nonnull_string (remote_protocol.c:30)
==6573==    by 0x4D28138: xdr_remote_domain_get_xml_desc_ret (remote_protocol.c:1418)
==6573==    by 0x4D3C0C2: virNetMessageDecodePayload (virnetmessage.c:382)
==6573==    by 0x4D3279F: virNetClientProgramCall (virnetclientprogram.c:382)
==6573==    by 0x4D0D50B: callWithFD (remote_driver.c:4339)
==6573==    by 0x4D0D5AB: call (remote_driver.c:4360)
==6573==    by 0x4D16EAF: remoteDomainGetXMLDesc (remote_client_bodies.h:861)
==6573==    by 0x4CF9F4F: virDomainGetXMLDesc (libvirt.c:4098)
==6573==    by 0x4154D9: cmdDomblklist (virsh.c:1722)
==6573==    by 0x4149E2: vshCommandRun (virsh.c:16365)
==6573==
==6573== 46,009 (352 direct, 45,657 indirect) bytes in 1 blocks are definitely lost in loss record 123 of 124
==6573==    at 0x4A05FDE: malloc (vg_replace_malloc.c:236)
==6573==    by 0x3318286DC6: xmlXPathNewContext (in /usr/lib64/libxml2.so.2.7.6)
==6573==    by 0x4C79AE2: virXMLParseHelper (xml.c:779)
==6573==    by 0x415512: cmdDomblklist (virsh.c:1726)
==6573==    by 0x4149E2: vshCommandRun (virsh.c:16365)
==6573==    by 0x427743: main (virsh.c:17867)
==6573==
==6573== LEAK SUMMARY:
==6573==    definitely lost: 2,188 bytes in 2 blocks
==6573==    indirectly lost: 45,657 bytes in 332 blocks
==6573==      possibly lost: 0 bytes in 0 blocks
==6573==    still reachable: 128,034 bytes in 1,364 blocks
==6573==         suppressed: 0 bytes in 0 blocks
Signed-off-by: NAlex Jia <ajia@redhat.com>

tools/virsh.c

@@ -1759,6 +1759,9 @@ cmdDomblklist(vshControl *ctl, const vshCmd *cmd)
 cleanup:
     VIR_FREE(disks);
     virDomainFree(dom);
+    VIR_FREE(xml);
+    xmlFreeDoc(xmldoc);
+    xmlXPathFreeContext(ctxt);
     return ret;
 }