fix(vm_image_util): Remove fstab from PXE initrd

Using fstab made overriding it with kernel command line options like
root=/some/disk not work properly. A generator handles it now.
Merge pull request #216 from skarllot/master

New script to create VirtualBox VDI
Merge pull request #217 from marineam/diskless

fix(vm_image_util): Remove fstab from PXE initrd
Small fix to contrib VirtualBox script

Fixed VirtualBox script to create final VDI file at correct location.
Removed the file extension from VirtualBox script.