If runqemu fails, ensure the log output is shown as its invaluable
to aid debugging. Its slightly convoluted since we need to ensure
we don't block on reading the pipe which may still be executing
hence the need for nonblocking IO.
(From OE-Core rev: 0e0fa1461863ec586b4f028dfd7d641f091ea928)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>