Give message to error when VGPU_type import fails

This was raising an error with an empty string. Now it follows the same pattern
as the error paths for importing other types (see GPU group import).

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
Merge pull request #1461 from simonjbeaumont/depantsify-import-error

Give message to error when VGPU_type import fails