First, we add your current user account to the operator group and we give the anyone in the operator group read-write access to the raw partition. In my case it’s /dev/disk0s4, but it might be different for you; mount the partition with Disk Utility and check the device with the mount command. We use VBoxManage to create vmdk wrapper for just partition 4 and make sure our regular user has access to the generated vmdk files. Add the top-level vmdk to your VM as a AHCI SATA drive; check SSD if applicable.

sudo dseditgroup -o edit -a $(whoami) -t user operator
sudo chmod 660 /dev/disk0s4
sudo echo chmod 660 /dev/disk0s4 > /etc/rc.local
sudo VBoxManage internalcommands createrawvmdk -rawdisk /dev/disk0 -filename Win8.1-bootcamp.vmdk -partitions 4
sudo chown $(whoami) Win8.1-bootcamp*.vmdk