xfs: support realtime/log device setup changes in config sections

xfs: support realtime/log device setup changes in config sections

Currently changing the devices used by "USE_EXTERNAL" environmental
variable is not supported by the config section parsing. Add the
functionality so that we can use config sections to test external
device configs successfully.

This required tracking down a bug in _check_xfs_filesystem() which
was causing a log device to be passed to a test device without an
external log device. This was caused by an uninitialised variable in
the function. I also added full output file removals to the first
couple of generic tests that were failing, because that's where the
check failure output ends up in this case.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>