start: Return error when failing to get IP

At the moment, nil is returned when Start() fails to gets the VM IP
crc's code assumes that when Start() returns, the VM has an IP.
This should help catch this situation.

This is related to crc-org/crc#2214

pkg/libvirt/libvirt.go

@@ -400,6 +400,7 @@ func (d *Driver) Start() error {
 
 	if d.IPAddress == "" {
 		log.Warnf("Unable to determine VM's IP address, did it fail to boot?")
+		return fmt.Errorf("Unable to determine VM's IP address, did it fail to boot?")
 	}
 	return nil
 }