sp: encapsulate spm status in StoragePool

The scope of this patch is to group the spm status values and access
them consistently in StoragePool and HSM.

Change-Id: I3b71349e2a0dfc453b68cd2d9ca6c563b1bee90c
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21527
Reviewed-by: Dan Kenigsberg <danken@redhat.com>