x86: prevent PVH Dom0 from having pages with more than one ref

x86: prevent PVH Dom0 from having pages with more than one ref

On PV guests a reference is taken when a page gets added to the page
tables, which makes pages added to the page tables have two
references, but this is not suitable for PVH that doesn't use the
PVMMU. In the PVH case only one reference has to be taken or else the
page would not be freed when the memory of the domain is decreased.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>