KVM: selftests: Rename vm_vcpu_add* helpers to better show relationships

Rename vm_vcpu_add() to __vm_vcpu_add(), and vm_vcpu_add_default() to
vm_vcpu_add() to show the relationship between the newly minted
vm_vcpu_add() and __vm_vcpu_add().

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