Define the ring buffer size as a constant expression because it should
not depend on the guest page size.
Signed-off-by: NMaya Nakamura <m.maya.nakamura@gmail.com>
Reviewed-by: NMichael Kelley <mikelley@microsoft.com>
Signed-off-by: NDmitry Torokhov <dmitry.torokhov@gmail.com>