KVM: SVM: Sync g_pat with guest-written PAT value

When hardware supports the g_pat VMCB field, we can use it for emulating
the PAT configuration that the guest configures by writing to the
corresponding MSR.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Tested-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>