vbox: Store VM files under ~/.capstan/instances/vbox

We will store all the files related to an instance under
~/.capstan/instances/vbox/$instance_id for vbox.
qemu: Allow multiple instances running at the same time

Create a new qcow image with the backing file pointing the original
image. This way, we will not change the original 'golden image' pulled
from remote repo.

Signed-off-by: Asias He <asias@cloudius-systems.com>
Merge pull request #42 from asias/cow

cow