(it did not find the differencing disk image and was not able
to discard it automatically).

Example: vm-start win10 --hd-autoreset=off
Let's say the VM crashes. If you want to recover and save your work you
can start the VM again with autoreset turned off.

Added --discard-state flag to run a snapshot-configured VM without restoring the saved state (but resetting the differential disk).
This is useful for running a VM for the first time, when the init is needed, but we want the full boot.

vm-start: fullscreen with vboxsdl is buggy on some systems (such as lab 130 machines). It's better to use virtualbox command until we find how to fix it

vm-start: fixed fullscreen variable check

vm-start: implemented flags: --fullscreen, --no-init, --no-run with getopt