Improve StoragePool.detachAllDomains() monitoring.

Change-Id: Ia48d103894c976bd32396a83da78f5175de73307
Signed-off-by: Eduardo <ewarszaw@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/13928
Reviewed-by: Ayal Baron <abaron@redhat.com>
Reviewed-by: Yeela Kaplan <ykaplan@redhat.com>
Tested-by: Elad Ben Aharon <eladba1990@gmail.com>