Added parameter to force request timeout on xen

Now, its possible to force request timeout by setting the parameter `xenserver_timeout` when openning connection

closes #1866

```ruby
@connection = Fog::Compute.new({
    :provider => 'XenServer',
    :xenserver_url => @host,
    :xenserver_username => @user,
    :xenserver_password => @pass,
    :xenserver_timeout => 300
})
```