Call virt.driver.destroy before deallocating network.

Call virt driver destroy before releasing the instance's
ip address, so that certain failures can be handled.
For instance, if destroy raises InstancePowerOffFailure,
do not deallocate network resources.

Fixes bug 1180178

Change-Id: I9bde984d2393bd73a73eae6065defb2ed7c01492
Merge "Call virt.driver.destroy before deallocating network."