KVM: selftests: Use vcpu_clear_cpuid_feature() to clear x2APIC

Add X86_FEATURE_X2APIC and use vcpu_clear_cpuid_feature() to clear x2APIC
support in the xAPIC state test.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20220614200707.3315957-30-seanjc@google.com