More convenient management of vm-0 state (`draksetup modify-vm0`)

[psrok1]

What I'm really missing is a common method to safely modify vm-0 without the risk of damaging the base snapshot.

My proposed solution is draksetup modify-vm0 utility that comes with the following subcommands:

• draksetup modify-vm0 begin - restores the vm-0 using a temporary snapshot, so changes are not affecting the base snapshot
• draksetup modify-vm0 commit - merges temporary snapshot into the vm-0 base image, JSON profiles are recreated if needed, vm-0 modification is done
• draksetup modify-vm0 rollback - temporary state of vm-0 is destroyed, vm-0 is effectively rollbacked to the previous base state without modification

It would be also a convenient way to recover from problems during draksetup postinstall execution.