Replace another Vchan.S.S with Vchan.S.ENDPOINT

Signed-off-by: David Scott <dave.scott@citrix.com>
Make {read,write}_size optional; and adopt a safe default of 1024

Ideally we would use a larger buffer size by default but this would
require multiple grants, and some contemporary linux kernel
xen-gnt{dev,alloc} devices can't handle multiple page grants/maps.

Signed-off-by: David Scott <dave.scott@citrix.com>