[Feature] mount guest disk

Often needed and might be useful to a lot more people.

Mount the guest disk:

uvt-kvm mount <guestname> <mountpoint>

This can do two things:
- if running it has all it needs for ssh, so you could just do sshfs mount
- more often people want/need to fix because it won't start so when the guest is off you could do like [1]. (btw I hate [2] as alternative as it never really works for me)

[1]: https://www.kumari.net/index.php/system-adminstration/49-mounting-a-qemu-image
[2]: http://ask.xmodulo.com/mount-qcow2-disk-image-linux.html