KVM: SVM: obey guest PAT

For many years some users of assigned devices have reported worse
performance on AMD processors with NPT than on AMD without NPT,
Intel or bare metal.

The reason turned out to be that SVM is discarding the guest PAT
setting and uses the default (PA0=PA4=WB, PA1=PA5=WT, PA2=PA6=UC-,
PA3=UC).  The guest might be using a different setting, and
especially might want write combining but isn't getting it
(instead getting slow UC or UC- accesses).

Thanks a lot to geoff@hostfission.com for noticing the relation
to the g_pat setting.  The patch has been tested also by a bunch
of people on VFIO users forums.

Fixes: 709ddebf
Fixes: https://bugzilla.kernel.org/show_bug.cgi?id=196409


Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Tested-by: Nick Sarnie <commendsarnex@gmail.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>