engine: Fix ClassCastException while attempting to detach direct LUN

Engine mistakenly trying to update VM snapshot id on direct LUN and
fails with ClassCastException. This patch restores code checking the
type of the disk before updating snapshot id.

Change-Id: I2b60d19f8832392d6f2df5cb05552560b7e10d1c
Bug-Url: https://bugzilla.redhat.com/1026920
Signed-off-by: Sergey Gotliv <sgotliv@redhat.com>