LVHDoISCSISR: simplify attach logic

pylint 2.13.9 seems to notice that `stored_exception` does not get set
if `self.iscsiSRs` is empty:

 drivers/LVHDoISCSISR.py:514:22: E0601: Using variable 'stored_exception' before assignment (used-before-assignment)

The code can indeed be simplified, as `connected` and
`stored_exception` were indeed tighly related, and only one is needed
in the end.

Signed-off-by: Yann Dirson <yann.dirson@vates.fr>

drivers/LVHDoISCSISR.py

@@ -491,7 +491,7 @@ def delete(self, sr_uuid):
 
     def attach(self, sr_uuid):
         try:
-            connected = False
+            stored_exception = None
             for i in self.iscsiSRs:
                 try:
                     i.attach(sr_uuid)
@@ -503,14 +503,12 @@ def attach(self, sr_uuid):
                         continue
                     else:
                         raise
-                else:
-                    connected = True
 
                 if not i._attach_LUN_bySCSIid(self.SCSIid):
                     raise xs_errors.XenError('InvalidDev')
 
             # Check if at least one iscsi succeeded
-            if not connected:
+            if stored_exception:
                 raise stored_exception
 
             if 'multiSession' in self.dconf: