Auto assign volume device name during attachment

In current powervc driver code, our sync code will always set the nova
instance root_device_name as None, this will break nova framework's
device name auto-assign function if the device name is not provide while
volume attachment. PowerVC itself will not set it's instance
root_device_name from it's implemenation, for PowerVC driver, we just
get the device name of the boot volume from the powervc side, and make
it as the root_device_name of the nova instance.

Closes-Bug: #1369577
Change-Id: I7e71573e7b83ec9f8c34cfa4f0a1736b90402a90