securable: refactor the securable implementation

In order to remove the storage pool metadata we need more flexibility
in defining the isSafe method.
In fact the different backends (with/without metadata) will rely on the
StoragePool SPMness to protect their methods.

In this patch:
- transform the Securable base class and MetaSecurable in a decorator

Change-Id: Id5a8be7536748481746795e27701fbecc7c3318c
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/22115
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Ayal Baron <abaron@redhat.com>