Plugin definition should match the actual gem name

This PR allows the use of `Vagrant.has_plugin?` like this:

```ruby
if Vagrant.has_plugin?('vagrant-vbguest')
  config.vbguest.auto_update = false
else
  abort(%Q|Please run 'vagrant plugin install --plugin-source https://rubygems.org/ --plugin-prerelease vagrant-vbguest'|)
end
```

Currently the only way to have this working is by matching the plugin definition `vbguest management`:

```ruby
if Vagrant.has_plugin?('vbguest management')
  config.vbguest.auto_update = false
else
  abort(%Q|Please run 'vagrant plugin install --plugin-source https://rubygems.org/ --plugin-prerelease vagrant-vbguest'|)
end
```

Thanks!