KVM: x86: Enable Intel AVX-512 for guest

Expose Intel AVX-512 feature bits to guest. Also add checks for
xcr0 AVX512 related bits according to spec:
http://download-software.intel.com/sites/default/files/managed/71/2e/319433-017.pdf

Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 7a9cb56..14c49cf 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -666,6 +666,12 @@
 	if ((!(xcr0 & XSTATE_BNDREGS)) != (!(xcr0 & XSTATE_BNDCSR)))
 		return 1;
 
+	if (xcr0 & XSTATE_AVX512) {
+		if (!(xcr0 & XSTATE_YMM))
+			return 1;
+		if ((xcr0 & XSTATE_AVX512) != XSTATE_AVX512)
+			return 1;
+	}
 	kvm_put_guest_xcr0(vcpu);
 	vcpu->arch.xcr0 = xcr0;