esx: Fix incorrect memory compare size in esxStoragePoolLookupByUUID

Use MD5_DIGEST_SIZE or VIR_UUID_BUFLEN rather than VIR_UUID_STRING_BUFLEN
when compare @uuid with @md5.

(cherry picked from commit 77cc51a4)

src/esx/esx_storage_backend_iscsi.c

@@ -220,7 +220,7 @@ esxStoragePoolLookupByUUID(virConnectPtr conn,
          target; target = target->_next) {
         md5_buffer(target->iScsiName, strlen(target->iScsiName), md5);
 
-        if (memcmp(uuid, md5, VIR_UUID_STRING_BUFLEN) == 0)
+        if (memcmp(uuid, md5, VIR_UUID_BUFLEN) == 0)
             break;
     }