target-xtensa: make 'sim' to be the default machine

This fixes the following error:

    $ qemu-system-xtensa -cpu help
    Segmentation fault
Signed-off-by: NMax Filippov <jcmvbkbc@gmail.com>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

hw/xtensa_sim.c

@@ -111,6 +111,7 @@ static void xtensa_sim_init(ram_addr_t ram_size,
 static QEMUMachine xtensa_sim_machine = {
     .name = "sim",
     .desc = "sim machine (" XTENSA_DEFAULT_CPU_MODEL ")",
+    .is_default = true,
     .init = xtensa_sim_init,
     .max_cpus = 4,
 };