vl: fix memory leak spotted by valgrind

valgrind complains about:
==9276== 13 bytes in 1 blocks are definitely lost in loss record 1,046 of 3,673
==9276==    at 0x4C2845D: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==9276==    by 0x2EAFBB: malloc_and_trace (vl.c:2556)
==9276==    by 0x64C770E: g_malloc (in /usr/lib64/libglib-2.0.so.0.3600.3)
==9276==    by 0x4A28BD: addr_to_string (vnc.c:123)
==9276==    by 0x4A29AD: vnc_socket_local_addr (vnc.c:139)
==9276==    by 0x4A9AFE: vnc_display_local_addr (vnc.c:3240)
==9276==    by 0x2EF4FE: main (vl.c:4321)
Signed-off-by: NShannon Zhao <zhaoshenglong@huawei.com>
Signed-off-by: NShannon Zhao <shannon.zhao@linaro.org>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

vl.c

@@ -4312,8 +4312,9 @@ int main(int argc, char **argv, char **envp)
     /* init remote displays */
     qemu_opts_foreach(qemu_find_opts("vnc"), vnc_init_func, NULL, 0);
     if (show_vnc_port) {
-        printf("VNC server running on `%s'\n",
-               vnc_display_local_addr("default"));
+        char *ret = vnc_display_local_addr("default");
+        printf("VNC server running on `%s'\n", ret);
+        g_free(ret);
     }
 #endif
 #ifdef CONFIG_SPICE