Event-channel setup: Re-bind if the connection becomes unbound (e.g.,
due to 'slow' domain suspend cancellation), even if the remote port
identifier has not changed.

Domain logging: Only open log file once (don't leak fds) and fix a
small memory leak.

Evtchn changes based on a patch by Jiri Denemark <jdenemar@redhat.com>

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