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>

docs/features/qemu-deprivilege.pandoc

@@ -66,6 +66,23 @@ this, create a user named `xen-qemuuser-shared`; for example:
 
     adduser --no-create-home --system xen-qemuuser-shared
 
+A final way to set up a separate process for qemus is to allocate one
+UID per VM, and set the UID in the domain config file with the
+`device_model_user` argument.  For example, suppose you have a VM
+named `c6-01`.  You might do the following:
+
+    adduser --system --no-create-home --group xen-qemuuser-c6-01
+
+And then in your config file, the following line:
+
+    device_model_user="xen-qemuuser-c6-01"
+
+NOTE: It is important when using `device_model_user` that EACH VM HAVE
+A SEPARATE UID, and that none of these UIDs map to root.  xl will
+throw an error a uid maps to zero, but not if multiple VMs have the
+same uid.  Multiple VMs with the same device model uid will cause
+problems.
+
 ## Domain config changes
 
 The core domain config change is to add the following line to the

tools/libxl/libxl_types.idl

@@ -495,7 +495,6 @@ libxl_domain_build_info = Struct("domain_build_info",[
     ("device_model",     string),
     ("device_model_ssidref", uint32),
     ("device_model_ssid_label", string),
-    # device_model_user is not ready for use yet
     ("device_model_user", string),
 
     # extra parameters pass directly to qemu, NULL terminated