KVM: selftests: Convert cpuid_test away from VCPU_ID

Convert cpuid_test to use vm_create_with_one_vcpu() and pass around a
'struct kvm_vcpu' object instead of using a global VCPU_ID.

Opportunistically use vcpu_run() instead of _vcpu_run(), the test expects
KVM_RUN to succeed.

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