Shadow: Make gfn_t always an unsigned long. GFNs are passed around

Shadow: Make gfn_t always an unsigned long.  GFNs are passed around
outside the shadow code, and although a 32-bit gfn_t is guaranteed to
hold all GFNs that can be found in a 32-bit pagetable, comparisons
with INVALID_GFN aren't safe when (-1UL) != (u32)(-1).
Signed-off-by: Tim Deegan <Tim.Deegan@citrix.com>