core: Edit USB policy only on down status

restrict editing the UsbPolicy field only when the vm is down.
this is because devices are reported from the VM after some period of
time which is not long but unpredicted which could lead to adding new
devices without detaching the old one.

Change-Id: Idb0698112b914b81599d87bbf45351c17c568602
Bug-Url: https://bugzilla.redhat.com/950768
Signed-off-by: Roy Golan <rgolan@redhat.com>