engine: Export/import monitors count fixed

Templates exported and imported back from the export storage domain
(using clone) end up with 0 monitors (VM cannot be created
from the template since we do not support headless vms).

The template to be exported didn't update managed devices from db
during the export and the XML didn't contain information about monitors.

Change-Id: Ie833ee6de83fad82932e252e8ed83df71b5c495c
Bug-Url: https://bugzilla.redhat.com/917375
Signed-off-by: Libor Spevak <lspevak@redhat.com>