docs: Fix dm_restrict documentation

docs: Fix dm_restrict documentation

Remove "chatty" and redundant information from the xl man page;
restrict it to functional descriptions only, and point instead to
qemu-depriv.pandoc and SUPPORT.md as locations for "canonical"
information.

Add a man page entry for device_model_user.

Update qemu-deprivilege.pandoc:

Changes in missing feature list:
- Migration is functional
- But qdisk backends are not

Add a missing restriction list.

The following statements from the man page are dropped:
- Mentioning PV; PV guests never have a device model.
- Drop the confusing statement about stdvga and cirrus vga options.
- Re-used domain IDs are now handled.
- Device models should no longer be able to create world-readable
  files on dom0's filesystem.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>