virtiofs_submounts.py test: Note on vmlinuz param

virtiofs_submounts.py test: Note on vmlinuz param

From the cancel message, it is not entirely clear why this parameter is
mandatory now, or that it will be optional in the future.  Add such a
more detailed explanation as a comment in the test source file.

Suggested-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20210212151649.252440-1-mreitz@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Cleber Rosa <crosa@redhat.com>