Change the console handling, to remove the loop and timeout if a console fails

Change the console handling, to remove the loop and timeout if a console fails
to respond first time.  For restored and unpaused domains there is no need for
the console to output data, so we cannot use this as a way to detect the
liveness of a console.  Instead, a command must always be sent to the console,
which means that this failure can only be detected once the constructor of the
Console class has returned successfully.  The __chewall loop still remains, in
order to detect runaway consoles and to clear out previous data from the
console before executing commands, but we no longer treat failure of that
method to read data as a failure of the console itself.

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