Removes the 'sharedfile' storage type

Since storage type are supposed to represent the underlying technology
of disk templates, the storage type 'sharedfile' is superfluous, because
technically both disk templates 'file' and 'sharedfile' use the file system.
This will be of use when implementing the storage space reporting for
all disk templates.

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Michele Tartara <mtarara@google.com>