migration: let incoming side use thread context

migration: let incoming side use thread context

The old incoming migration is running in main thread and default
gcontext.  With the new qio_channel_add_watch_full() we can now let it
run in the thread's own gcontext (if there is one).

Currently this patch does nothing alone.  But when any of the incoming
migration is run in another iothread (e.g., the upcoming migrate-recover
command), this patch will bind the incoming logic to the iothread
instead of the main thread (which may already get page faulted and
hanged).

RDMA is not considered for now since it's not even using the QIO watch
framework at all.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180502104740.12123-2-peterx@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>