Change how event channels are allocated and used by the control

Change how event channels are allocated and used by the control
tools. /dev/xen/evtchn is now used by daemons to connect to
remote domains: the advantage is that the local ports are garbage
collected automatically if the daemon dies. xen no longer
constructs end-to-end event-channel port pairs -- it allocates an
unbound port in new domU and writes that port to xenstore. It is
then picked up by teh appropriate daemon which does interdomain bind
via /dev/xen/evtchn.

Signed-off-by: Keir Fraser <keir@xensource.com>