KVM: vmx: obey KVM_QUIRK_CD_NW_CLEARED

OVMF depends on WB to boot fast, because it only clears caches after
it has set up MTRRs---which is too late.

Let's do writeback if CR0.CD is set to make it happy, similar to what
SVM is already doing.
Signed-off-by: Xiao Guangrong <guangrong.xiao@intel.com>
Tested-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx.c

@@ -8650,6 +8650,9 @@ static u64 vmx_get_mt_mask(struct kvm_vcpu *vcpu, gfn_t gfn, bool is_mmio)
 
 	if (kvm_read_cr0(vcpu) & X86_CR0_CD) {
 		ipat = VMX_EPT_IPAT_BIT;
+		if (kvm_check_has_quirk(vcpu->kvm, KVM_QUIRK_CD_NW_CLEARED))
+			cache = MTRR_TYPE_WRBACK;
+		else
 			cache = MTRR_TYPE_UNCACHABLE;
 		goto exit;
 	}