pool: refactor getInfo and getRepoStats

Both getInfo and getRepoStats (in StoragePool) grown some
inconsistencies over time. It's time to do some cleanup in
this area before proceeding with any additional change.

Change-Id: Ib151b9d19ea488ebbe4c0921a797ddda08141e49
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/10304
Reviewed-by: Ayal Baron <abaron@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>