OvmfPkg: VirtioFlush(): return the number of bytes written by the host

VirtioLib provides an API for simple, synchronous (request/response-style)
virtio communication. The guest driver builds one descriptor chain, link
for link, with VirtioPrepare() and VirtioAppendDesc(), then submits the
chain, and awaits the processing, with VirtioFlush().

The descriptor chain is always built at the beginning of the descriptor
area, with the head descriptor having descriptor index 0.

In order to submit the descriptor chain to the host, the guest always
pushes a new "available element" to the Available Ring, in genuine
queue-like fashion, with the new element referencing the head descriptor
(which always has index 0, see above).

In turn, after processing, the host always pushes a new "used element" to
the Used Ring, in genuine queue-like fashion, with the new element
referencing the head descriptor of the chain that was just processed. The
same element also reports the number of bytes that the host wrote,
consecutively across the host-writeable buffers that were linked by the
descriptors.

(See "OvmfPkg/VirtioNetDxe/TechNotes.txt" for a diagram about the
descriptor area and the rings.)

Because at most one descriptor chain can be in flight with VirtioLib at
any time,

- the Available Ring and the Used Ring proceed in lock-step,

- and the head descriptor that the new "available" and "used" elements can
  ever reference has index 0.

Based on the above, we can modify VirtioFlush() to return the number of
bytes written by the host across the descriptor chain. The virtio-block
and virtio-scsi drivers don't care (they have other ways to parse the data
produced by the host), while the virtio-net driver doesn't use
VirtioFlush() at all (it employs VirtioLib only to set up its rings).

However, the virtio entropy device,  to be covered in the upcoming
patches, reports the amount of randomness produced by the host only
through this quantity.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

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