Just setting the max threads to 0 is enough to stop all workers.

Message-Id: <20220514065012.1149539-4-pbonzini@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>