core: prevent spmStop on CannotRemoveLogicalVolume

Prevent spmStop on CannotRemoveLogicalVolume VDSM error.

Change-Id: Ibc06dedc6f62647a8060f9284574d512d1890bd2
Bug-Url: https://bugzilla.redhat.com/878938
Signed-off-by: Daniel Erez <derez@redhat.com>