fix qdev_print_devinfo()

snprintf returns number of bytes needed for the output, not the number
of bytes actually written.  Thus the math is wrong ...

Spotted by Markus Armbruster.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Message-Id: 
