scripts: Print number of threads in 'osv info threads'

The id of the last thread might mislead people's thinking about
the number of threads in OSv, so let's simply print the nr after all
threads were already printed.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>