qemu-iotests: Add _unsupported_fmt helper

This helper allows adding tests supporting any format expect the
specified formats. This may be useful to test that many formats behave
in a common way.

Signed-off-by: Nir Soffer <nirsof@gmail.com>
Message-id: 20170201003120.23378-3-nirsof@gmail.com
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>