vhost: drop VIRTIO_F_NOTIFY_ON_EMPTY feature

This feature is not particularly useful for storage.
It forces us to read additional vq memory on each
I/O completion and that's quite expensive.

Quoting VIRTIO 1.0:

```
VIRTIO_F_NOTIFY_ON_EMPTY (24)
If this feature has been negotiated by driver, the device MUST issue an
interrupt if the device runs out of available descriptors on a
virtqueue, even though interrupts are suppressed using the
VIRTQ_AVAIL_F_NO_INTERRUPT flag or the used_event field.
```

Later on:

```
Note: An example of a driver using this feature is the legacy networking
driver: it doesn’t need to know every time a packet is transmitted, but
it does need to free the transmitted packets a finite time after they
are transmitted. It can avoid using a timer if the device interrupts it
when all the packets are transmitted.
```

Change-Id: I7f53293bf811a4cd5ae8e42e18f35042ea6f4ba8
Suggested-by: Pawel Wodkowski <pawelx.wodkowski@intel.com>
Signed-off-by: Dariusz Stojaczyk <dariuszx.stojaczyk@intel.com>
Reviewed-on: https://review.gerrithub.io/398325


Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Pawel Wodkowski <pawelx.wodkowski@intel.com>
Reviewed-by: Daniel Verkamp <daniel.verkamp@intel.com>