Fixes instance task_state being left as migrating

In a cross-hypervisor environment, such as KVM + Hyper-v. After negative test
which live migrate a KVM instance to Hyper-v host, there will be an exception
InvalidHypervisorType. As a result, the instance task_state is being
left as migrating. That leaves the instance in a broken state and most of the
requests on the instance will fail.

Actually, there are some other exceptions will cause this issue as well, such
as UnableToMigrateToSelf, DestinationHypervisorTooOld. All of them are thrown
from the driver of scheduler during live migration. But obviously, if the live
migration fails at the schedule stage, the task_state of the target instance
should be updated to None.

Fixes Bug: 1153283

Change-Id: Ide4b48653268c8dbe31de6964814830d2e82a5ba
Add missing API doc for networks-post-req.

Part of the os-tenant-networks API extension.

Fixes LP Bug #1155303.

Change-Id: Ic71a3d108f9401ec81d2c59a6f7ba3ec6ab59dd8
Merge "Add missing API doc for networks-post-req."
Merge "Fixes instance task_state being left as migrating"