x86: populate PVHv2 Dom0 physical memory map

x86: populate PVHv2 Dom0 physical memory map

Craft the Dom0 e820 memory map and populate it. Introduce a helper to remove
memory pages that are shared between Xen and a domain, and use it in order to
remove low 1MB RAM regions from dom_io in order to assign them to a PVHv2 Dom0.

On hardware lacking support for unrestricted mode also craft the identity page
tables and the TSS used for virtual 8086 mode.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>