We currently check if the VDDK image is specified for a vmware provider but we don't check the availability of the image when it's specified. We should fail the validation of the provider in this case (otherwise, we fail later on when reaching the conversion phase).

This should be verified by (ordered by importance):

1. Specifying an image that doesn't contain VDDK as the VDDK init image and check that the plan is set with a condition that says "VDDK init image is invalid"
2. Specifying a non-existing image as the VDDK init image and check that after 5 minutes the plan is set with a condition that says "VDDK init image is invalid"