core: NPE in maintenanceNumOfVds command

Due to a recent change.
Also since the update is always static false, there's
no need to fetch the entity prior to the update.

Change-Id: Ia916d44fb2ab23d34c99ef5096e4d9e193541fba
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>