stats: compute the iso prefix asynchronously

The iso prefix must be computed asynchronously because in any other
synchronous operation (e.g.: connectStoragePool, getInfo) vdsm cannot
risk to stop and wait for the iso domain to report its prefix (since
it might be unreachable).

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=880098
Change-Id: I09b7be78da544547a6ed9d5b440bdb0037004c16
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/10305
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>