exec: Protect map_client_list with mutex

So that accesses from multiple threads are safe.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <1426496617-10702-4-git-send-email-famz@redhat.com>
[Remove #if from cpu_exec_init_all. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
