xenbus: Improvements to wait_for_devices().

xenbus: Improvements to wait_for_devices().
 1. When printing a warning about a timed-out device, print the
 current state of both ends of the device connection (i.e., backend as
 well as frontend).
 2. A device is 'not yet connected' only when the local state is *less
 than* XenbusStateConnected. If the state is Closing or Closed
 (usually because of an explicit failure when trying to make the
 connection) then we should not wait for the connection to occur -- it
 will never happen!

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