To allow user to use RAW disks /f.e. vmdk/ in VirtualBox, you must run:
VBoxManage internalcommands createrawvmdk -filename /path/to/file.vmdk -rawdisk /dev/sda -partitions 1,5 -relative
usermod -a -G disk 'user_name'
See more at https://www.virtualbox.org/manual/ch09.html#rawdisk