xen: build on ARM

Collection of fixes to build QEMU with Xen support on ARM:
- use xenstore_read_fe_uint64 to retrieve the page-ref (xenfb);
- use xen_pfn_t instead of unsigned long in xenfb;
- unsigned long/xenpfn_t in xen_remove_from_physmap;
- in xen-mapcache.c use HOST_LONG_BITS to check for QEMU's address space
size.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
