Handle RAM more gracefully

Currently the VMs start with 1GiB of RAM. The memory can be increased to 4GiB by issuing:

```
virsh --connect qemu:///system qemu-monitor-command --domain vixos_firefox --hmp 'balloon 4096'
```

But that's not automatic, and by default the VM will stay with 1 GiB. And 1 GiB is not much, browser will hang with a few tabs open.

We should implement something to autobaloon the VMs.