libxl: use correct type modifier for vuart_gfn

libxl: use correct type modifier for vuart_gfn

Fixes compilation error like:

libxl_console.c: In function ‘libxl__device_vuart_add’:
libxl_console.c:379:5: error: format ‘%lu’ expects argument of type ‘long unsigned int’, but argument 3 has type ‘xen_pfn_t’ [-Werror=format=]
      flexarray_append(ro_front, GCSPRINTF("%lu", state->vuart_gfn));

Reported-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Tested-by: Bhupinder Thakur <bhupinder.thakur@linaro.org>