qemu-iotests: support per-format golden output files

Some tests produce format-dependent output.  Either the difference is
filtered out and ignored, or the test case is format-specific so we
don't need to worry about per-format output differences.

There is a third case: the test script is the same for all image formats
and the format-dependent output is relevant.  An ugly workaround is to
copy-paste the test into multiple per-format test cases.  This
duplicates code and is not maintainable.

This patch allows test cases to add per-format golden output files so a
single test case can work correctly when format-dependent output must be
checked:

  123.out.qcow2
  123.out.raw
  123.out.vmdk
  ...

This naming scheme is not composable with 123.out.nocache or 123.pc.out,
two other scenarios where output files are split.  I don't think it
matters since few test cases need these features.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Message-id: 20170705125738.8777-9-stefanha@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
