Some more advanced devices like VirtIO GPU do expect they can receive
responses to VirtIO commands they issue via VirtIO queue. Such responses
are not sent via a separate device writeable queue. Instead the driver
is expected to queue some additional descriptors and buffers which the
device can then use to provide the reply.
This patch adds support for such write-data-read-response opeartion to
Genode VirtIO::Queue implementation. The implementation is pretty simple
and does not support any fancy features like receiving the response
asynchronously. Instead the operation will use caller provided callback
to wait for the device to process the command. Once this callback
returns the write-data-read-response VirtIO::Queue function will invoke
another callback passing received response as argument.
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