VMware: raise an exception for unsupported disk formats

At the moment 'vmdk' and 'iso' are the only disk formats supported.
If an invalid format is passed with the image then the driver will
throw and exception.

Change-Id: I834856f33a0644bc03b89a2b98e4e188c112e338
Closes-bug: #1287844
Merge "VMware: raise an exception for unsupported disk formats"