oeqa/qemurunner: Fix binary vs str issue
The recent logging changes for qemurunner showed up as errors on the
autobuilder where decode couldn't be called on the returned string.
Since the code returns binary data, return b'' instead of '' to match
to avoid tracebacks.
One of these cases was newly added, copied from the other which has
been there for a long time, always broken.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit
b8995b27db265b0a0b2d2ca595915f70f9f96e07)
Signed-off-by: Steve Sakoman <steve@sakoman.com>
