libxl: Resolve possible resource leak in dom0 maximum memory setting

If either the "if (STRPREFIX(mem_tokens[j], "max:"))" is never entered
or the "if (virStrToLong_ull(mem_tokens[j] + 4, &p, 10, maxmem) < 0)" break
is hit, control goes back to the outer loop processing 'cmd_tokens' and
it's possible that the 'mem_tokens' would be overwritten.

Found by Coverity

src/libxl/libxl_conf.c

@@ -1622,6 +1622,7 @@ libxlDriverGetDom0MaxmemConf(libxlDriverConfigPtr cfg,
                 goto cleanup;
             }
         }
+        virStringListFree(mem_tokens);
     }
 
  physmem: