Improved the method to list all virtual machines

A new method has been added to the Abiquo 2.4 api to provide direct
access to all the virtual machines available to the user. This makes
unnecessary to iterate over all virtual datacenters and appliances of
that user.

The strategy doing that has been removed, and the new method has been
added to the AsyncApi. The unit tests for this method have also been
implemented as expect tests.