doc: add code example for virtio-user exception path

doc: add code example for virtio-user exception path

The HOWTO guide for using virtio-user as an exception path to the kernel
only provided an example of how testpmd may be used for that purpose.
However, a real application wanting to use virtio-user as exception path
would likely want to create such devices from code within the app
itself. Therefore, we update the doc with instructions and a code
snippet showing how this may be done.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Chenbo Xia <chenbo.xia@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>