Add en locale for long_shutoff message

Running a vagrant status mentions long_shutoff is missing:

```
jenkins@rts50:~/foo$ vagrant status
Current machine states:

slave1                    shutoff (libvirt)

translation missing: en.vagrant_libvirt.states.long_shutoff
jenkins@rts50:~/foo$ vagrant plugin list
vagrant-libvirt (0.0.19)
  - Version Constraint: 0.0.19
vagrant-login (1.0.1, system)
vagrant-mutate (0.3.0)
vagrant-share (1.1.0, system)
```