qemurunner.py: print output from runqemu/qemu-system in stop()

This is done when starting up qemu has failed, but is not done
when qemu started ok, but fails later in QMP communication.

Output from runqemu does contain valuable information to find out
why, so rather than fix all the QMP fails to include it, let's just
print it in stop().

Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 6e2bf68e)
Signed-off-by: Steve Sakoman <steve@sakoman.com>