Don't prematurely fake transmit responses

Don't prematurely fake transmit responses

If the device is being ejected then the backend state will transition from
XenbusStateConnected to XenbusStateClosing. This is done by the toolstack
and should have no effect on the backend's ability to complete requests.
Therefore, when we fake responses in __TransmitterRingDisable() we could
be racing with an active backend. Hence amend the test in the loop to
consider a backend in either XenbusStateConnected or XenbusStateClosing to
be worth waiting for.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>