Allow the addition of more disks to a VM

[jhkrischel]

We'd like to be able to add additional disks - example code for virtualbox:

        driver.execute("createmedium", "disk", "--filename", disk_file, "--size", "#{disk_size}", "--format", "VDI")
        driver.execute('storageattach', uuid, '--storagectl', "SATA Controller", '--port', "#{sata_port}", '--type', 'hdd', '--medium', disk_file)

[spazquest]

Added to Skytap's internal Jira as ENG-33289

[lfirwin]

Ability added in latest pull request. Doesn't quite meet the original request above, but does create additional disks for a VM given a list of disk sizes. For example, adding 2 additional disks to a VM of 25GB in size:

box.disks = [25600,25600]