add support for additional disks in VMs

[ooraini]

A really simple implemention for additional disks. I used a dict for the config key disks for extensibility. Currently only qcow2 disks using the default pool can be created.

[goneri]

Thank you for the patch @ooraini. I like the feature idea. It's a nice addition that will be useful.

[goneri]

Thank you @ooraini :heart:!