KVM: selftests: Use kvm_cpu_has() for XSAVES in XSS MSR test

Use kvm_cpu_has() in the XSS MSR test instead of open coding equivalent
functionality using kvm_get_supported_cpuid_index().

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20220614200707.3315957-10-seanjc@google.com