Which `vboxmanage clonehd` variant corresponds to the GUI selection `dynamically allocated`?
According to this forum post:


Standard (the default) - one piece dynamic (sparse) extent file, plus a small descriptor file.
Fixed - extents will be flat (rather than dynamic/sparse).
Split2G - image is split into multiple extents, each less than 2GB.
Stream - image is optimized for streaming downloads rather than running in a VM. This type may be compressed.
ESX ?


so standard is you proper option or no --variant at all since standard is the default.