Reset task_state when resetting vm_state to ACTIVE

A virt driver can raise InstanceFaultRollback to indicate that the
instance should be reset to ACTIVE.  However, the code that resets to
ACTIVE does not reset task_state to None.

This patch fixes that.  Technically, there's currently no bug here,
because the compute manager methods that matter here all have the
reverts_task_state() decorator to always reset task_state to None on
exceptions.  However, a bug would be introduced later if we remove
those decorators.

Change-Id: Ibcf8ae3473f601781d8aae80f88df3da54bbaac8
hyperv: Move tests under nova/tests/virt/hyperv

Change-Id: Ia4ea16e5aa4a48e1f10a52394f7b59fdfdb68d54
Merge "hyperv: Move tests under nova/tests/virt/hyperv"
Merge "Reset task_state when resetting vm_state to ACTIVE"