unicore32-softmmu: Add is_default setting for puv3 machine

This patch sets is_default to 1 for puv3 machine, so that
find_default_machine() returns puv3 machine.
Thanks Dunrong for pointing it out.

Cc: Dunrong Huang <riegamaths@gmail.com>
Signed-off-by: Guan Xuetao <gxt@mprc.pku.edu.cn>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

hw/puv3.c

@@ -119,6 +119,7 @@ static QEMUMachine puv3_machine = {
     .name = "puv3",
     .desc = "PKUnity Version-3 based on UniCore32",
     .init = puv3_init,
+    .is_default = 1,
     .use_scsi = 0,
 };