Fix restart of VMs
Fix restart of VMs

We can't keep appending to self.qemu_args in start() since it'll build an
incorrect command string on the second run, as is the case with a restart of a
VM, if it should fail for any reason.

bdreisbach/vrnetlab,holmahenkel/vrnetlab,bdreisbach/vrnetlab,plajjan/vrnetlab,mzagozen/vrnetlab,mzagozen/vrnetlab,plajjan/vrnetlab,holmahenkel/vrnetlab