Package machines as plain VMs, not templates

[legal90]

Our official boxes and all other boxes created by Packer already contains a plain VM image inside.

But vagrant package command clones VM to template prior to packaging to the box. Actually, there is no any significant reason for this. Templates are the same as "plain" VM images except they could not be configured and started.

However, it causes a problem when we try to create a linked clone from such box - It fails because it's not possible to snapshot a VM template (it is "read-only").

So, I've decided to use only plain VMs inside boxes:

1. On Import action we should check the VM image type. If it is a template, then convert it to plain VM (it's extremely fast and should be done only once). This helps to keep backward compatibility - all previously created boxes will work fine.
2. On Export action we should clone VM to the VM, not to template

[legal90]

Implemented by #227