kvm tools: Return number of bytes written by term_putc

No caller is currently using the return value but better return
number of bytes written instead of 0 in case of an error.

Cc: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Andreas Herrmann <andreas.herrmann@caviumnetworks.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>