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>