Improve gandi vm images commands to also display available existing disk images
Fixes bug with configuration file not truncated upon save and becoming corrupted.

Flag was missing so if we wrote data smaller than previous we had junk at the end of file.
vm: don't store system disk size in configuration file

It fixes the following error starting from an empty configuration file:
Error: Invalid value for "--size": s is not a valid integer

Reviewed-by: sayoun <sayoun@gandi.net>
Signed-off-by: William Dauchy <william@gandi.net>