devicedisk: Don't set driver name=unknown for rbd vols

tests/cli-test-xml/compare/virt-install-many-devices.xml

@@ -72,7 +72,7 @@
       <target dev="vdf" bus="virtio"/>
     </disk>
     <disk type="volume" device="disk">
-      <driver name="qemu" type="unknown"/>
+      <driver name="qemu"/>
       <source volume="some-rbd-vol" pool="rbd-ceph"/>
       <target dev="vdg" bus="virtio"/>
     </disk>

virtinst/diskbackend.py

@@ -552,7 +552,9 @@ class StorageBackend(_StorageBase):
 
     def get_driver_type(self):
         if self._vol_object:
-            return self.get_vol_xml().format
+            ret = self.get_vol_xml().format
+            if ret != "unknown":
+                return ret
         return None
 
     def validate(self, disk):