selftest/qemurunner: Work around possible control character contamination

selftest/qemurunner: Work around possible control character contamination

Using a binary string as the login banner search expression is fraught with
risks. We've seen cases on the autobuilder where "login:" is clearly shown
but the code hasn't triggered. The most likely cause is hidden control characters
in the output causing the search to fail.

Take the opportunity to remove the horrible binary string search, at the expense of
decoding the bootlog multiple times.

Tweak the logging so we can know which log was printed (self.msg or bootlog)
just in case this isn't the issue and we need more information in future.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>