KVM: selftests: Rename xAPIC state test's vcpu struct

Rename xapic_state_test's kvm_vcpu struct to xapic_vcpu to avoid a
collision when the common 'struct vcpu' is renamed to 'struct kvm_vcpu'
in a future patch.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>