KVM: VMX: check use I/O bitmap first before unconditional I/O exit

According to Table C-1 of Intel SDM 3C, a VM exit happens on an I/O instruction when
"use I/O bitmaps" VM-execution control was 0 _and_ the "unconditional I/O exiting"
VM-execution control was 1. So we can't just check "unconditional I/O exiting" alone.
This patch was improved by suggestion from Jan Kiszka.

Reviewed-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Zhihui Zhang <zzhsuny@gmail.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

arch/x86/kvm/vmx.c

@@ -6521,11 +6521,8 @@ static bool nested_vmx_exit_handled_io(struct kvm_vcpu *vcpu,
 	int size;
 	u8 b;
 
-	if (nested_cpu_has(vmcs12, CPU_BASED_UNCOND_IO_EXITING))
-		return 1;
-
 	if (!nested_cpu_has(vmcs12, CPU_BASED_USE_IO_BITMAPS))
-		return 0;
+		return nested_cpu_has(vmcs12, CPU_BASED_UNCOND_IO_EXITING);
 
 	exit_qualification = vmcs_readl(EXIT_QUALIFICATION);