xen: Console.connect will now block waiting for console hotplug

It's convenient to block in a Xenstore watch rather than immediately
fail because this allows stand-alone console backends to be used,
such as xentropyd.

An application can still choose to die if it doesn't have immediate
access to a console, or die after a timeout, or sit and wait.

Signed-off-by: David Scott <dave.scott@citrix.com>