add a snapshot feature

[nongiach]

Add a snapshot feature
arm_now snapshot -h
snapshot create [NAME]: create snapshot of the current VM
snapshot list: list all available snapshot
snapshot use [NAME]: overwrite the current VM with the snapshot [NAME]

use case

$ arm_now start x86-i686
# do some config
# poweroff
$ arm_now snapshot create MyBestVM
$ arm_now snapshot list
MyBestVM: maybe add a description here
$ arm_now start
# run some malwares or not trustworthy things
# poweroff
$ arm_now snapshot use MyBestVM
$ arm_now start
# Just go back in time

[SakiiR]

I'll try to do that this weekend my man :)