imgconverter: ensure that connection to the VM is properly closed

Even in case of some error (for example invalid image).

(cherry picked from commit fc3347c914)

core/imgconverter.py

@@ -152,8 +152,9 @@ get_from_stream(), get_from_vm(), get_xdg_icon_from_vm(), get_through_dvm()'''
         p.stdin.write('{0}\n'.format(src))
         p.stdin.close()
 
+        try:
             img = cls.get_from_stream(p.stdout, **kwargs)
-
+        finally:
             p.stdout.close()
         if p.wait():
             raise Exception('Something went wrong with receiver')