xen/mm: Clarify the granularity for each Frame Number

xen/mm: Clarify the granularity for each Frame Number

ARM64 is able to support 64KB and 4KB page granularities. While the guest
will support both granularities, Xen and hypercall interface will always
be in 4KB.

Signed-off-by: Stefano Stabellini <stefano.stabellini@citrix.com>
Signed-off-by: Julien Grall <julien.grall@citrix.com>
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Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Keir Fraser <keir@xen.org>
Cc: Tim Deegan <tim@xen.org>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
I'm missing one term for Linux Frame Number but always in 4KB
granularity. It's necessary in few places such as the balloon code where
we need to map a Linux 4KB Frame Number into a Machine Frame Number.

I was thinking to name it xen_pfn.