util: Implement qemu_get_thread_id() for OpenBSD

Implement qemu_get_thread_id() for OpenBSD hosts, using
getthrid().
Signed-off-by: NDavid Carlier <devnexen@gmail.com>
Reviewed-by: NBrad Smith <brad@comstyle.com>
Message-id: CA+XhMqxD6gQDBaj8tX0CMEj3si7qYKsM8u1km47e_-U7MC37Pg@mail.gmail.com
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
[PMM: tidied up commit message]
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

util/oslib-posix.c

@@ -100,6 +100,8 @@ int qemu_get_thread_id(void)
     return (int)tid;
 #elif defined(__NetBSD__)
     return _lwp_self();
+#elif defined(__OpenBSD__)
+    return getthrid();
 #else
     return getpid();
 #endif