fixes import for hyper-v generation 2 VMs

Currently, packaging a Hyper-V Generation 2 VM and running `Vagrant UP` will result in a VM being created that cannot boot into the OS and the `vagrant up` command will throw since it cannot discover the guest IP.

This simply finds the `subtype` of the vm in the vm config which is the zero based generation and adds the `Generation` parameter to the `New-VM` cmdlet passing the one-based value.

This can be tested using:
```
vagrant box add mwrock/Windows8.1-amd64 --provider hyperv
```