drivers/xenfront-xenbus: Fix scanf() variable type

Spotted by GCC with '-Wformat=1':

../../drivers/xenfront-xenbus.cc: In function ‘XenbusState xenbus_get_state(device_t)’:
../../drivers/xenfront-xenbus.cc:219:80: warning: format ‘%d’ expects argument of type ‘int*’, but argument 6 has type ‘XenbusState* {aka xenbus_state*}’ [-Wformat=]
     xs_scanf(XST_NIL, dev->get_node_path().c_str(), "state", NULL, "%d", &state);

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>