Add a small intro + minimal documentation for how to
implement SR/IOV support for an emulated device.

Signed-off-by: Knut Omang <knuto@ifi.uio.no>
Message-Id: <20220217174504.1051716-3-lukasz.maniak@linux.intel.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>