dm_depriv: Describe expected usage of device_model_user parameter

dm_depriv: Describe expected usage of device_model_user parameter

A number of subsequent patches rely on as-yet undefined behavior for
what the `device_model_user` parameter does.  Rather than implement it
incorrectly (or randomly), or remove the feature, describe an expected
usage for the feature.  Further patches will make decisions based on
this expected usage.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>