oeqa/ssh: Improve performance and log sizes

oeqa/ssh: Improve performance and log sizes

The current code is not fit for purpose when handling large files via ssh. In the strace
ptest case, we can end up with a 1.4GB archive being transferred for which every
byte is printed into the task logfile twice over. This is then sent over bitbake IPC
which compounds the problems.

Make the following improvements:
  * when the output is large (over 64kb), don't print it
  * use a bytearray for better concat performance since strings are slow for this
  * when there is no ssh output, say that
  * print periodic size status output rather than the data itself since this could be binary and/or large
  * fix the killed process message logic which appeared broken

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit e7dd009a17dc902852983a82bce41bf78bb1e242)
Signed-off-by: Steve Sakoman <steve@sakoman.com>