t5868: VirtualBox issue on copying vdi files and other improvements

Using the same SDK image to setup old and new VM instance results in a VirtualBox error:

Locking of attached media failed. A possible reason is that one of the media is attached to a running VM.
Result Code:
VBOX_E_INVALID_OBJECT_STATE (0x80BB0007)
Component:
SessionMachine
Interface:
IMachine {85cd948e-a71f-4289-281e-0ca7ad48cd89}

This commit change the test case to clone the VM instead of creating two identical VMs.

This patch also fixes other issues that were pointed out by:

https://phabricator.apertis.org/T5868#195314

Signed-off-by: Peter Senna Tschudin peter.senna@collabora.com