core, restapi,engine: Avoid enabling balloon device on VM edit

Avoids the creation of a balloon device on every REST API call that
changes any of attibute of a VM.

Change-Id: I81ba4e286eab5195b763843cd1adaaeb273070de
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1152991
Signed-off-by: Vitor de Lima <vdelima@redhat.com>