Remove the bizarre arrangement whereby EventChannel inherits from dict. Move

Remove the bizarre arrangement whereby EventChannel inherits from dict.  Move
the channel-creation logic into eventChannel, making the EventChannel class
simpler.  Remove the closeEventChannel method -- it was simply doing a check
for None, and that isn't enough value to justify the extra method when there
is an EventChannel.close method there too.

Signed-off-by: Ewan Mellor <ewan@xensource.com>