fix(linstor): use quotes on linstor type annotations (#150)

Without that and if LINSTOR python package is not installed we can have
issues to register our drivers. `AttributeError` is raised and not catched by
other modules during imports...

Trace:
```
Jul 30 17:11:36 hpm1c20 xapi: [error||0 |Registering SMAPIv1 plugins D:6829eea78bbd|sm_exec] Rejecting SM plugin: LVM because of exception: Storage_error ([S(Backend_error);[S(SR_BACKEND_FAILURE);[S(non-zero exit);S();S(Traceback (most recent call last):\x0A  File "/opt/xensource/sm/LVMSR", line 23, in <module>\x0A    import SR\x0A  File "/opt/xensource/sm/SR.py", line 23, in <module>\x0A    import VDI\x0A  File "/opt/xensource/sm/VDI.py", line 21, in <module>\x0A    import cleanup\x0A  File "/opt/xensource/sm/cleanup.py", line 44, in <module>\x0A    import blktap2\x0A  File "/opt/xensource/sm/blktap2.py", line 60, in <module>\x0A    from linstorvolumemanager import get_controller_uri, get_all_volume_openers, LinstorVolumeManager\x0A  File "/opt/xensource/sm/linstorvolumemanager.py", line 262, in <module>\x0A    class LinstorVolumeManager(object):\x0A  File "/opt/xensource/sm/linstorvolumemanager.py", line 420, in LinstorVolumeManager\x0A    def native_client(self) -> linstor.Linstor:\x0AAttributeError: module 'linstor' has no attribute 'Linstor'\x0A)]]]) (executable)
```

Signed-off-by: Ronan Abhamon <ronan.abhamon@vates.tech>

Author: Wescoeur

drivers/linstorvolumemanager.py

@@ -417,7 +417,7 @@ def uri(self) -> str:
         return self._uri
 
     @property
-    def native_client(self) -> linstor.Linstor:
+    def native_client(self) -> "linstor.Linstor":
         return self._linstor
 
     @property