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>