Show XML error message, fix for #88

Here is a fix for #88.
In case of HTTP 400 response, it will parse the XML response.body and read the message attribute and append that to the message raised.

The error from #88 now looks like this, even without debug logging:

```
C:\jenkins\workspace\windows_2008_r2-100gb_vcloud\boxtest>vagrant up --provider=vcloud
Bringing machine 'default' up with 'vcloud' provider...
==> default: Building vApp...
==> default: vApp windows_2008_r2-100gb-vagrant-vmware-slave-615ef4a5 successfully created.
C:/Users/vagrant/.vagrant.d/gems/gems/vagrant-vcloud-0.4.0/lib/vagrant-vcloud/driver/base.rb:356:in
`send_request': Warning: unattended code 400 Bad Request: Guest customization requires at least version
7299 of VMware tools. You can deselect 'Enable guest customization' for now and select it after installing
VMware tools to perform guest customization. (RuntimeError)
```

Now it is easier to find what caused the problem.

I have also added the intense logging if debug logging is turned on, so also there the response appears with header and body.