Make use of pthread_set_name_np() to be able to set the threads name
on OpenBSD.

Signed-off-by: Brad Smith <brad@comstyle.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <Y57NrCmPTVSXLWC4@humpty.home.comstyle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>