VMDK opening fails when parentFileNameHint is set to full path

[object1337]

When attempting to mount a VMDK file that was created using thin provisioning, it fails with a file not found error on the parent VMDK
file, due to it attempting to find it in the same directory. This is caused by the parentFileNameHint that is set to the full path of the parent VMDK file, but ignored by dissect.

I've create a pull request that resolves this issue

[object1337]

unit test added to pull request