sp: stop all poolMonitoredDomains on disconnect

When we attempt to stop the DomainMonitor threads in StoragePool
we should use the poolMonitoredDomains list instead of using
getDomains.
Beside being more consistent and reliable it also fixes a race
in destroyStoragePool where we detach the last domain and then
we attempt to stop the remaining DomainMonitor.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1059757
Change-Id: Ibc77988b01491f687fd7afa6bb34512b5f2451af
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/24088
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Sergey Gotliv <sgotliv@redhat.com>
Reviewed-by: Ayal Baron <abaron@redhat.com>