KVM: selftests: Ensure KVM supports AVX for SEV-ES VMSA FPU test

Verify that KVM's supported XCR0 includes AVX (and earlier features) when
running the SEV-ES VMSA XSAVE test.  In practice, the issue will likely
never pop up, since KVM support for AVX predates KVM support for SEV-ES,
but checking for KVM support makes the requirement more obvious.

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://lore.kernel.org/r/20241003234337.273364-12-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>