There are a couple places (one generic, one target specific) where we need
to get the host page size associated with a particular memory backend.  I
have some upcoming code which will add another place which wants this.  So,
for convenience, add a helper function to calculate this.

host_memory_backend_pagesize() returns the host pagesize for a given
HostMemoryBackend object.
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>
Reviewed-by: NGreg Kurz <groug@kaod.org>
Acked-by: NPaolo Bonzini <pbonzini@redhat.com>