x86: Emulate MTRR enable/disable

x86: Emulate MTRR enable/disable

We assume that cells will only flip the enabled flag of
IA32_MTRR_DEF_TYPE, leaving the rest of the register in default state
(the one found during handover). SVM already implemented this but
emulated the disabled state by modifying the host PAT.

This approach works less invasively by only changing the effective guest
PAT to 0 in case MTRRs are off. And it provides this for Intel as well.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>