Signals the hot-unplugging of a virtual (guest) CPU.

Signed-off-by: Lluís Vilanova <vilanova@ac.upc.edu>
Message-id: 148278748597.1404.10546320797997984932.stgit@fimbulvetr.bsc.es
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>