qemu-doc: deprecate query-cpus

Start the deprecation period for QAPI query-cpus (replaced by
query-cpus-fast) beginning with 2.12.0.

Signed-off-by: Viktor Mihajlovski <mihajlov@linux.vnet.ibm.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1518797321-28356-5-git-send-email-mihajlov@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>