KVM: selftests: Verify XCR0 can be "downgraded" and "upgraded"

Now that KVM selftests enable all supported XCR0 features by default, add
a testcase to the XCR0 vs. CPUID test to verify that the guest can disable
everything except the legacy FPU in XCR0, and then re-enable the full
feature set, which is kinda sorta what the test did before XCR0 was setup
by default.

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://lore.kernel.org/r/20241003234337.273364-7-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

tools/testing/selftests/kvm/x86_64/xcr0_cpuid_test.c

@@ -79,6 +79,11 @@ static void guest_code(void)
 	ASSERT_ALL_OR_NONE_XFEATURE(supported_xcr0,
 				    XFEATURE_MASK_XTILE);
 
+	vector = xsetbv_safe(0, XFEATURE_MASK_FP);
+	__GUEST_ASSERT(!vector,
+		       "Expected success on XSETBV(FP), got vector '0x%x'",
+		       vector);
+
 	vector = xsetbv_safe(0, supported_xcr0);
 	__GUEST_ASSERT(!vector,
 		       "Expected success on XSETBV(0x%lx), got vector '0x%x'",