libxl: Fix the bug introduced in commit "libxl: use correct type modifier for vuart_gfn"

libxl: Fix the bug introduced in commit "libxl: use correct type modifier for vuart_gfn"

In libxl__device_vuart_add vuart_gfn is getting stored as a hex value:

> flexarray_append(ro_front, GCSPRINTF("%"PRI_xen_pfn, state->vuart_gfn));

However, xenstore reads this value as a decimal value and tries to map the
wrong address and fails.

This patch introduces a new format specifier "PRIu_xen_pfn" which formats the value as a
decimal value.

Signed-off-by: Bhupinder Thakur <bhupinder.thakur@linaro.org>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Release-acked-by: Julien Grall <julien.grall@linaro.org>