qemu backend - dnsmasq leases don't expire, ip addresses 

Creating many different VM's each with a unique name, eventually makes dnsmasq exhaust its ip address pool.

The addresses don't expire and are not released when a vm instance is stopped or even purged.

it can be reproduced with the following:

```
#!/bin/bash
for i in {1..512}
do
  bin/multipass launch -n test${i}
  bin/multipass delete -p test${i}  
done
```