core: return lver/spm id from pool metadata

Currently the lver/spm id are being returned as they appear in the
cluster lock if supported.

The fenceSpmStorage verb is currently used to manually confirm that
there's no current spm on the responsibillity of the caller. This verb
updates the lver/spm id in the pool metadata to indicate that the spm
role is free.

This won't help us currently as getSpmStatus will return the info from
the cluster lock (if supported) which wasn't edited at all. As currently
we can't edit the information return from cluster lock, we can return
the same results as in case it wasn't supported by the cluster lock.

Right now the solution is implemented in StoragePoolDiskBackend only as
a bug fix, as StoragePoolMemoryBackend isn't being used at the moment.

Change-Id: I460801329a9a1c5ee940bce22566ad3d29b351de
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1092631
Signed-off-by: Liron Aravot <laravot@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27226
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>