KVM: VMX: enable VMXON check with SMX enabled (Intel TXT)

Per document, for feature control MSR:

  Bit 1 enables VMXON in SMX operation. If the bit is clear, execution
        of VMXON in SMX operation causes a general-protection exception.
  Bit 2 enables VMXON outside SMX operation. If the bit is clear, execution
        of VMXON outside SMX operation causes a general-protection exception.

This patch is to enable this kind of check with SMX for VMXON in KVM.

Signed-off-by: Shane Wang <shane.wang@intel.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

include/linux/tboot.h

@@ -150,6 +150,7 @@ extern int tboot_force_iommu(void);
 
 #else
 
+#define tboot_enabled()			0
 #define tboot_probe()			do { } while (0)
 #define tboot_shutdown(shutdown_type)	do { } while (0)
 #define tboot_sleep(sleep_state, pm1a_control, pm1b_control)	\