core: allowing to remove external VMs from the engine

This patch splits the commands allowed on non engine-managed VMs to two different
sets:
1. Commands for external VMs
2. Commands for hosted engine VMs

A difference, for example, is that we allow to stop and remove external VMs, but not hosted
engine VMs.

Change-Id: I69eeeaaa933f8b8eeb9a984c91cba06d8d05ebe1
Bug-Url: https://bugzilla.redhat.com/978268
Signed-off-by: Oved Ourfali <oourfali@redhat.com>