exec: Introduce qemu_maxrampagesize() and rename qemu_getrampagesize()

Rename qemu_getrampagesize() to qemu_minrampagesize(). While at it,
properly rename find_max_supported_pagesize() to
find_min_backend_pagesize().

s390x is actually interested into the maximum ram pagesize, so
introduce and use qemu_maxrampagesize().

Add a TODO, indicating that looking at any mapped memory backends is not
100% correct in some cases.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20190417113143.5551-3-david@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>