trace: Lock vCPU list when initializing dynamic tracing state

Fixes potential corruption when a vCPU is hot-(un)plugged while
initializing the current one.

Signed-off-by: Lluís Vilanova <vilanova@ac.upc.edu>
Message-id: 148278747515.1404.6538173443841279200.stgit@fimbulvetr.bsc.es
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
