CA-141611: move parsing of the request to a separate function

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
CA-141611: write I/O barriers might contain data

Write I/O barriers might contain data to be written, so they must be
treated as a special kind of write.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Signed-off-by: Felipe Franciosi <felipe.franciosi@citrix.com>
CA-141611: address review comments

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>