Fix AttributeError in RPC code for DVR

Fix the usage of call methods as per recent changes according to
drop-rpc-compat. These changes went overlooked and it broke DVR.

Tests will be done as follow-up.

Partial-bug: #1394848

Change-Id: I6e0584f8e54e606a76b87853b2371cc8e24eba69
Merge "Fix AttributeError in RPC code for DVR"
Merge branch 'master' of https://github.com/openstack/quantum