pvh/dom0: warn when dom0_mem is not set

pvh/dom0: warn when dom0_mem is not set

There have been several reports of the dom0 builder running out of
memory when building a PVH dom0 without having specified a dom0_mem
value. Print a warning message if dom0_mem is not set when booting in
PVH mode.

This is a temporary workaround until accounting for internal memory
required by Xen (ie: paging structures) is improved.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>