prevent deadlocks during event stream teardown

[mjdwitt]

previous to this change, Libvirt.SubscribeQEMUEvents sometimes failed
to shut down on context cancellation due to a race between two
incorrectly-ordered defers:

ctx, cancel := context.WithCancel(ctx)
defer cancel()
defer l.unsubscribeQEMUEvents(stream)
defer stream.Shutdown()

the deferred stream.Shutdown would run first, allowing a small window
where the stream was closed but still registered to receive events. if an
event arrived in that window, the shared socket-reader goroutine (which
routes every incoming event and RPC response for the connection) would
block forever inside Stream.Push, since nothing was left to drain the
stream's queue. because that goroutine is shared across the whole
connection, blocking it wedged every other in-flight RPC too - including
the deregister call l.unsubscribeQEMUEvents(stream) was waiting on, and
Disconnect's own close call. this change reorders those defers so the
stream is always unsubscribed before its local reader is shut down,
closing the window.

furthermore, this also adds a guard in Stream.Push that prevents
hanging if the stream has already been shut down via context
cancellation.

[mpontillo]

Looks good; let's credit the original author (as in the AUTHORS change in #190), otherwise lgtm!