fix(linstor): handle RAW type in compute_volume_size (#158)

Regression caused by this commit:
```
refactor(sm): move all VDI types in a new module `vditype.py`
```

The RAW type is no longer checked by this function;
this prevents the creation of RAW volumes and consequently blocks HA activation.

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

Author: Wescoeur

drivers/linstorcowutil.py

@@ -474,7 +474,7 @@ def compute_volume_size(self, virtual_size: int) -> int:
             )
             bitmap_overhead = self._cowutil.calcOverheadBitmap(virtual_size)
             virtual_size += meta_overhead + bitmap_overhead
-        else:
+        elif self._vdi_type != VdiType.RAW:
             raise Exception('Invalid image type: {}'.format(self._vdi_type))
 
         return LinstorVolumeManager.round_up_volume_size(virtual_size)