allow disk update on previously created VM

[treywelsh]

Allow to attach/detach disks to a running/poweroff VM.

The implementation take the decision based on the comparison of the image_id argument between the new disks configured, and the disks in the terraform state.

Operations are done in this order:

1. If an image_id is found in the state, but not in the new config, it's considered as attached, thus, the disk will be detached based on the disk_id attribute (which is the disk number)
2. If an image_id is find in the new config, but not in state, this image ID will be attached

Between each operations, wait on the state to be back on Running/poweroff again, and check disk presence (attach method is async)

These are the states from which it's possible to call attach/detach actions.
So, by the way I updated and refactored the method to wait on VM state, and for consistency, the method to wait on image state.

PR: #59

Feel free to drop a comment if you see some problem with this behavior

[treywelsh]

PR #59 merged.

Side note: later work related to issue #71 may conflict with this PR.