iscsi_direct: Reset pool capacity and allocation just before refresh
    
    Jirka reported a bug that with every 'virsh pool-refresh' an
    iscsi-direct pool would grow and grow. The problem is that
    virISCSIDirectRefreshVol() only adds to def->capacity and
    def->allocation but nothing clears it out to begin with.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
    Reviewed-by: Pavel Hrdina <phrdina@redhat.com>