get_model method missing for Ploop image

Image.get_model is called in partition injection code,
and now inject partition attempt fails unconditionally.
This patch makes use of disk/api.py inject_data failure tolerance:
it doesn't fail unless injected data is mandatory.

Closes-Bug: 1670642
Change-Id: I6c68693be818219f78d6fcda019b867066396b29

nova/virt/libvirt/imagebackend.py

@@ -1100,6 +1100,9 @@ class Ploop(Image):
                                        target,
                                        out_format)
 
+    def get_model(self, connection):
+        return imgmodel.LocalFileImage(self.path, imgmodel.FORMAT_PLOOP)
+
 
 class Backend(object):
     def __init__(self, use_cow):