[Load Balancer] Adding VMs via CLI does not work

> ### `az feedback` auto-generates most of the information requested below, as of CLI version 2.0.62

**Describe the bug**

Currently it is not possible to add VMs to the backends of Load Balancers using their VM/NIC IDs, but by an IP address. Whereas on the Console it is possible to do exactly this. Adding VMs by their IP fails to add them to the load balancer.
**To Reproduce**

Create a LB via az cli, adding VMs via their IPs.
**Expected behavior**

VMs be found by their IP, or allow the ability to specifiy VM NICs to allow adding of VMs this way.
**Environment summary**

Install via https://docs.microsoft.com/en-us/cli/azure/install-azure-cli?view=azure-cli-latest,
```
az --version
azure-cli 2.12.0

core 2.12.0
telemetry 1.0.6

Python location '/opt/az/bin/python3'
Extensions directory '/home/jujumanage/.azure/cliextensions'

Python (Linux) 3.6.10 (default, Sep 22 2020, 05:20:19)
[GCC 7.5.0]

Legal docs and information: aka.ms/AzureCliLegal

Your CLI is up-to-date.
```
**Additional context**