libxl: dm_restrict: Move to domain_build_info

libxl: dm_restrict: Move to domain_build_info

Right now, this is broken because libxl__build_device_model_args_new
is used also for the qemu run for pv guests for qdisk devices, pvfb,
etc.

We can either make this option properly HVM-specific, or make it
generic.

In principle it is a reasonable request, to make the PV qemu
deprivileged (even though it is not likely to be implemented any time
soon).  So make this option generic.

We retain the name "device model" even though it is arguably
inaccurate, because the xl docs already say, for example
  For a PV guest a device-model is sometimes used to provide backends
  for certain PV devices

The documentation patch here is pure code motion.  For ease of review
we will fix up the docs, so the wording to be right for the new
context, in the next patch.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Reported-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
---
v2: Change xl too, to avoid breaking the build.
    Fix manpage pod syntax.