fix: patching the path of Async block devices

The asynchronous engine maintains an event file descriptor which passes
to the IO uring interface when creating a new ring. IO uring uses this
EventFd to notify us about completion of IO requests.

When we PATCH an async block device, we create a new asynchronous
engine, including a new EventFd. However, we still monitor the old
EventFd. This breaks the use of async drives post PATCH requests,
because we never get notified about the results of requests we submit to
the IO uring engine.

This commit changes the implementation along the PATCH code path, to
reuse the previous EventFd for the asynchronous engine.

Signed-off-by: Babis Chalios <bchalios@amazon.es>