libxl: Avoid advertising about device_model_user config option

libxl: Avoid advertising about device_model_user config option

Running QEMU as non-root user is not ready yet, so replace the warning
with a debug message and remove the option from the man page.

Also improve the doc to include more potential issue with running QEMU
as non-root.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>