x86: constrain MFN range Dom0 may access

x86: constrain MFN range Dom0 may access

... to that covered by the physical address width supported by the
processor. This implicitly avoids Dom0 (accidentally or due to some
kind of abuse) passing out of range addresses to a guest, which in
turn eliminates this only possibility for PV guests to create PTEs
with one or more reserved bits set.

Note that this is not a security issue due to XSA-77.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>