Add missing unit attention ack and retry

Before reading registrations.
Without this ack, some systems report systematically this error on stop:

	E | Persistent reservation in:
	E | Fixed format, current; Sense key: Unit Attention
	E | Additional sense: Reservations preempted
	E | PR in (Read keys): Unit attention
	E | sg_persist failed: Unit attention

opensvc/drivers/resource/disk/scsireserv/sg.py

@@ -124,6 +124,7 @@ def read_registrations(self):
                 n_registered += out.count(self.hostid)
             else:
                 for path in paths:
+                    self.ack_unit_attention(path)
                     ret, out, err = self.read_path_registrations(path)
                     if ret != 0:
                         continue