mpty arguments for avocado.virt.qemu.machine.VM class will not raise exception later.

Allows to use `avocado.virt.qemu.machine.VM` with empty parameters, so you can make use of it in command line, like:

```
>>> import avocado.virt.qemu import machine
>>> vm = machine.VM()
>>> vm.power_on()
>>> vm.power_off()
```

without raising exception of kind 'NoneType' object and 'get' attribute.

Signed-off-by: Rudá Moura <rmoura@redhat.com>