libvirtio: lazy create the Event object in drain()

The drain method uses an asyncio.Event object to be notified when other
coroutines have removed all registered callbacks. The Event object needs
to be associated with the coroutine that the event loop is running with
and currently this is achieved by passing in the 'loop' parameter.

Unfortunately Python 3.10 has removed the 'loop' parameter and now the
object is associated implicitly with the current thread's event loop.
At the time the virEventAsyncIOImpl constructor is called, however,
there is no guarantee that an event loop has been set for the thread.
The explicitly passed in 'loop' parameter would handle this scenario.

For portability with Python >= 3.10 we need to delay creation of the
Event object until we have a guarantee that there is a loop associated
with the current thread. This is achieved by lazily creating the Event
object inside the 'drain' method, which is expected to be invoked from
coroutine context and thus ensure a loop is associated.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

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