KVM: selftests: Add check of CR0.TS in the #NM handler in amx_test

Be extra paranoid and assert that CR0.TS is clear when verifying the #NM
in the AMX test is due to the expected XFeature Disable error, i.e. that
the #NM isn't due to CR0.TS=1.

Signed-off-by: Mingwei Zhang <mizhang@google.com>
Link: https://lore.kernel.org/r/20230221163655.920289-6-mizhang@google.com
[sean: reword changelog to make it clear this is pure paranoia]
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: mzhang3579

tools/testing/selftests/kvm/x86_64/amx_test.c

@@ -216,6 +216,7 @@ void guest_nm_handler(struct ex_regs *regs)
 {
 	/* Check if #NM is triggered by XFEATURE_MASK_XTILEDATA */
 	GUEST_SYNC(7);
+	GUEST_ASSERT(!(get_cr0() & X86_CR0_TS));
 	GUEST_ASSERT(rdmsr(MSR_IA32_XFD_ERR) == XFEATURE_MASK_XTILEDATA);
 	GUEST_SYNC(8);
 	GUEST_ASSERT(rdmsr(MSR_IA32_XFD_ERR) == XFEATURE_MASK_XTILEDATA);