chardev: use per-dev context for io_add_watch_poll

It was only passed in by chr_update_read_handlers().  However when
reconnect, we'll lose that context information.  So if a chardev was
running on another context (rather than the default context, the NULL
pointer), it'll switch back to the default context if reconnection
happens.  But, it should really stick to the old context.

Convert all the callers of io_add_watch_poll() to use the internally
cached gcontext.  Then the context should be able to survive even after
reconnections.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <1505975754-21555-4-git-send-email-peterx@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>