Marking del/zero volumes in blockSD class.

Previous delete was not atomic since lvremove of multiple LVs can
partially fail (e.g. since volume is open) leaving some volumes
intact and others deleted.
Marking before delete will enable garbage collection in such cases.
The old rename convention is not atomic since the rename should be
done sequentially.

Related to: BZ#905038, BZ#910013, BZ#875708

Change-Id: I747bc2218e0b2cc256dd352ad890143c7d354bd8
Signed-off-by: Eduardo <ewarszaw@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/12545
Reviewed-by: Ayal Baron <abaron@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>