core: prevent changing display settings when vm isn't down

This patch adds display protocol and number of monitors to the
group of vm properties that can be updated only when the vm is
in down state, in order to prevent scanario in which the user
change them while the vm is in hibernate mode and expects the
vm to have the updated settings when the vm is restored from
hibernation.

Change-Id: I7b1676e8e0d69fa6579d3b2323d1372e2e55c4ef
Bug-Url: https://bugzilla.redhat.com/877938
Signed-off-by: Arik Hadas <ahadas@redhat.com>