dmx: More const correctness

Fixes several dozen cases like:

../hw/dmx/examples/ev.c: In function ‘main’:
../hw/dmx/examples/ev.c:147:29: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
                         tmp = "X";
                             ^

Signed-off-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Eric Anholt <eric@anholt.net>

hw/dmx/examples/ev.c

@@ -56,7 +56,7 @@ main(int argc, char **argv)
     int fd = 0;
     int rc;
     int i, j;
-    char *tmp;
+    const char *tmp;
 
 #define test_bit(bit) (mask[(bit)/8] & (1 << ((bit)%8)))