virtio-9p: Fix syntax error in debug code

This error was reported by cppcheck:

qemu/hw/9pfs/virtio-9p-debug.c:342:
error: Invalid number of character ({) when these macros are defined:
'DEBUG_DATA'.

Cc: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
