Added --extra-vdis option to the 'vm create' command

The option currently accepts the following formats:

* --extra-vdis 10,20
  Creates 2 additional VDIs (10 and 20 MB) using the default storage
  repository from the first pool found.

* --extra-vdis 'Local storage':10,'SSD storage':20
  Creates a 10MB VDI in the 'Local storage' storage repository and
  a 20MB VDI in 'SSD storage' storage repository.