The hypercall interface is always using 4KB page granularity. This is
requiring to use xen page definition macro when we deal with hypercall.
Note that pfn_to_gfn is working with a Xen pfn (i.e 4KB). We may want to
rename pfn_gfn to make this explicit.
We also allocate a 64KB page for the shared page even though only the
first 4KB is used. I don't think this is really important for now as it
helps to have the pointer 4KB aligned (XENMEM_add_to_physmap is taking a
Xen PFN).