Fix updating device names when defaulting

If the virt driver does not support defaulting device names, the default
behaviour of defaulting them in the manager was calling the conductor
block_device_mapping_update method with the wrong arguments, which
caused a TypeError and ultimately a failure to boot the instance.

This patch fixed the signature and adds a test that exercises it so
that it does not regress.

Closes-bug: #1260806
Change-Id: I6116540c9aca4847afacf80b3d1e0c027c364159
Merge "Fix updating device names when defaulting"