Feature Request: Read configuration from file

command line:
```bash
$ vmcli --config ubuntu/vm.conf
```

```text
shutdown-timeout=15
cpu-count=1
memory-size=256
kernel=kernel
initrd=rootfs
disk=disk-0.img
disk=disk-1.img
network=e6:64:3d:68:5c:78@nat
cmdline=console=hvc0 irqfixup root=/dev/vda
```
