The created VM images appear to have VT-x turned on; there is no way to turn this off, as the setting to do so is disabled unless the CPU supports VT-x (this seems like a VBox bug, but it apparently dates back at least 2 years). The command line tool does not appear to work either. There is no way to use the VMs at all, as the setting cannot be turned off by any means I could find; if theres a workaround, I'd love to hear it.
The only workaround I could find was to simply delete the VMs, create them again (using the existing disk images), then create new snapshots for each.
Latest version requires VirtualBox 5.0. Reopen if this issue persists.