debugcon: make debug message more readable

before change:
Bdebugcon: write addr=0x0000 val=0x6f
odebugcon: write addr=0x0000 val=0x6f
odebugcon: write addr=0x0000 val=0x74
tdebugcon: write addr=0x0000 val=0x69
idebugcon: write addr=0x0000 val=0x6e
ndebugcon: write addr=0x0000 val=0x67
gdebugcon: write addr=0x0000 val=0x20
 debugcon: write addr=0x0000 val=0x66

after change:
B [debugcon: write addr=0x0000 val=0x6f]
o [debugcon: write addr=0x0000 val=0x6f]
o [debugcon: write addr=0x0000 val=0x74]
t [debugcon: write addr=0x0000 val=0x69]
i [debugcon: write addr=0x0000 val=0x6e]
n [debugcon: write addr=0x0000 val=0x67]
g [debugcon: write addr=0x0000 val=0x20]
  [debugcon: write addr=0x0000 val=0x66]
Signed-off-by: Nliguang <lig.fnst@cn.fujitsu.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

hw/char/debugcon.c

@@ -55,7 +55,7 @@ static void debugcon_ioport_write(void *opaque, hwaddr addr, uint64_t val,
     unsigned char ch = val;
 
 #ifdef DEBUG_DEBUGCON
-    printf("debugcon: write addr=0x%04" HWADDR_PRIx " val=0x%02x\n", addr, val);
+    printf(" [debugcon: write addr=0x%04" HWADDR_PRIx " val=0x%02x]\n", addr, val);
 #endif
 
     qemu_chr_fe_write(s->chr, &ch, 1);