engine: Report error only for non-pluggable devices

Error should be reported (as it states in the error itself) only for non
pluggable devices, otherwise if the device is pluggable it's cool that
it is unplugged in DB and VDSM.

Change-Id: Id68197668fc2432bb7c26e6940fe8bf92bd3e674
Bug-Url: https://bugzilla.redhat.com/872165
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>