Run Ubuntu installed on external hard drive as guest OS

I am new to Ubuntu and have installed it on an external hard drive attached to my MacBook Pro Retina running OS X 10.9 and VMware Fusion 6.

I can boot into it plugging a USB drive with rEFInd on it, but I'd like to be able to have it in a VM running on my Mac.

I know I can create a VM from the install ISO, but that would take precious internal hard drive space, that I'd rather save.

Any ideas on what to do?  Is this even possible?  I thought running extFS (got the demo/trial) but it doesn't mount the main Linux LVM drive.

Thanks in advance for any help.