Move call to _default_block_device_names() inside try block

During instance builds, a call to _default_block_device_names() is
made which contains a db call to update the instance. In case the
instance is gone it will throw an unhandled InstanceNotFound exception.

To properly handle this, move the call to _default_block_device_names()
inside the existing try block next to it that handles this exception.

Change-Id: I3503029f02669e6ae31441653096fa69cba76d45
Closes-Bug: #1224677