dataplane: remove EventPoll in favor of AioContext

dataplane: remove EventPoll in favor of AioContext

During the review of the dataplane code, the EventPoll API morphed itself
(not concidentially) into something very very similar to an AioContext.
Thus, it is trivial to convert virtio-blk-dataplane to use AioContext,
and a first baby step towards letting dataplane talk directly to the
QEMU block layer.

The only interesting note is the value-copy of EventNotifiers.  At least
in my opinion this is part of the EventNotifier API and is even portable
to Windows.  Of course, in this case you should not close the notifier's
underlying file descriptors or handle with event_notifier_cleanup.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>