x86/ept: force WB cache attributes for grant and foreign maps

x86/ept: force WB cache attributes for grant and foreign maps

Force WB type for grants and foreign pages. Those are usually mapped
over unpopulated physical ranges in the p2m, and those ranges would
usually be UC in the MTRR state, which is unlikely to be the correct
cache attribute. It's also cumbersome (or even impossible) for the
guest to be setting the MTRR type for all those mappings as WB, as
MTRR ranges are finite.

Note that this is not an issue on AMD because WB cache attribute is
already set on grants and foreign mappings in the p2m and MTRR types
are ignored. Also on AMD Xen cannot force a cache attribute because of
the lack of ignore PAT equivalent, so the behavior here slightly
diverges between AMD and Intel (or EPT vs NPT/shadow).

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