Don't overwrite instance object with dict in _init_instance()

If a compute service is started while it has an instance in task_state
REBOOT_STARTED or REBOOT_STARTED_HARD it will trigger an instance update
which will overwrite the instance object with an instance dict.  This
causes an AttributeError on the next line when instance.task_state is
referenced.  This seems like a simple oversight and should be switched
to using the instance.save method.

Change-Id: I44c3eab2c8821b66b0b9ef4696e56bd577ae8fed
Closes-Bug: 1309067
(cherry picked from commit b4b5254a56d808f2582784897364de1adafeb43a)