[virtio] implement pop_or_enable_notification()
[virtio] implement pop_or_enable_notification()

This method makes it easy and efficient to make sure that we enable
notifications after processing all the available buffers.

Signed-off-by: Serban Iorga <4cd75ae1c2f5ea126376f5daa1ce9f5f3016cb24@amazon.com>

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