Clean up some unused wrap_exception() stuff

The only use of wrap_exception() without notifier and publisher_id
arguments is in nova/virt/libvirt/driver.py ... and since no notifier
argument is supplied, this decorator does exactly nothing useful. So
let's delete the useless uses of this in the libvirt driver.

Now we can make the notifier and publisher_id the only arguments to the
decorator and make them mandatory, allowing us to clean up some unused
stuff.

blueprint: oslo-messaging
Change-Id: I482e7f5e536493c06334b6aefb50c0e48d80c478
Merge "Clean up some unused wrap_exception() stuff"