x86/EPT: split super pages upon mismatching memory types

x86/EPT: split super pages upon mismatching memory types

... between constituent pages. To indicate such, the page order is
being passed down to the vMTRR routines, with a negative return value
(possible only on order-non-zero pages) indicating such collisions.

Some code redundancy reduction is being done to ept_set_entry() along
the way, allowing the new handling to be centralized to a single place
there.

In order to keep ept_set_entry() fast and simple, the actual splitting
is being deferred to the EPT_MISCONFIG VM exit handler.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Tim Deegan <tim@xen.org>
Acked-by: Kevin Tian <kevin.tian@intel.com>