KVM: selftests: Convert kvm_clock_test away from VCPU_ID

Convert kvm_clock_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() with an open
coded assert that KVM_RUN succeeded.

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