vhost-user: document memory accesses

vhost-user: document memory accesses

The vhost-user protocol specification does not define "guest address"
and "user address".  It does not explain how to access memory given such
addresses.

This patch explains how memory access works, including the IOTLB.

Cc: Michael S. Tsirkin <[email protected]>
Cc: Maxime Coquelin <[email protected]>
Cc: Wei Wang <[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>
Acked-by: Maxime Coquelin <[email protected]>