There's a couple of places that seem to duplicate this calculation
of RAM size above the 4G boundary. Move all those to a helper function.

Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20220719170014.27028-5-joao.m.martins@oracle.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>