core: importing external and hosted-engine VMs to the engine

This patch adds two new origin types:
1. Hosted engine - VMs that were created by the hosted-engine setup.
2. External - VMs that run on the engine hosts, but weren't created by
any oVirt project

Change-Id: I6b933df39aaf6897a675aef0564d3fb4922f12e7
Signed-off-by: Oved Ourfali <oourfali@redhat.com>