Add source dev parsing for vdpa interfaces

This change extends the guest xml parsing such that
the source device path can be extreacted from interface
elements of type vdpa.

This is required to identify the interface to remove when
detaching a vdpa port from a domain.

This change fixes a latent bug in the libvirt fixutre
related to the domain xml generation for vdpa interfaces.

Change-Id: I5f41170e7038f4b872066de4b1ad509113034960

tox.ini

@@ -34,7 +34,13 @@ extras =
   hyperv
   vmware
 passenv =
-  OS_DEBUG GENERATE_HASHES
+  OS_DEBUG
+  GENERATE_HASHES
+# NOTE(sean-k-mooney) optimization is enabled by default and when enabled
+# asserts are complied out. Disable optimization to allow asserts in
+# nova to fire in unit and functional tests. This can be useful for
+# debugging issue with fixtures and mocks.
+  PYTHONOPTIMIZE
 # there is also secret magic in subunit-trace which lets you run in a fail only
 # mode. To do this define the TRACE_FAILONLY environmental variable.
 commands =