koops: don't analyze MCEs like standard oopses

MCEs are not kernel oopses as we know them, MCEs do not have stacktrace
and ABRT drops oopses without meaningful stack trace frames for hashing.
The required files are generated by abrt-action-check-oops-for-hw-errror
because that executable understands the format of MCE data.
  - uuid    (sha1 of backtrace file)
  - duphash (sha1 of backtrace file)
  - kernel  (from dmesg)

Signed-off-by: Jakub Filak <jfilak@redhat.com>