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
(cherry picked from commit dacb187fcd685c15fd084fdfce8c5e165130ff54)
Merge "Call virt.driver.destroy before deallocating network." into stable/grizzly