xen: flush queue when getting an event

xen: flush queue when getting an event

xen does not have a register that, when written, will cause can_receive
to go from false to true.  However, flushing the queue can be attempted
whenever the front-end raises its side of the Xen event channel.  There
is a single event channel for tx and rx.

Cc: Stefano Stabellini <[email protected]>
Cc: Stefan Hajnoczi <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>
Reviewed-by: Amos Kong <[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>