oeqa/qemurunner: Clean up failure handling

If you fail to setup the tap devices, runqemu will error quickly
however stdout/stderr are not shown to the user, instead a SystemExit
traceback is shown. This could explain some long since unexplained
failures on the autobuilder.

Rework the error handling so SystemExit isn't used and the
standard log failure messages can be shown. The code could
likely ultimatley need some restructuring to work effectively.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Steve Sakoman <steve@sakoman.com>