Preserve the HTTP headers in Client, and set the interfaces and volumes in VM if possible.

The http_headers helper method in rbovirt.rb would overwrite the :accept header passed in by the Client#vm method, cutting out information (e.g. in my case the interfaces and disk volumes) the client would have to retrieve in additional API calls.

With these changes I was able to cut down listing machines (~8) in https://github.com/apache/deltacloud from 40 seconds to 5 seconds.