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>

tests/acceptance/virtiofs_submounts.py

@@ -228,6 +228,18 @@ class VirtiofsSubmountsTest(BootLinux):
     def setUp(self):
         vmlinuz = self.params.get('vmlinuz')
         if vmlinuz is None:
+            """
+            The Linux kernel supports FUSE auto-submounts only as of 5.10.
+            boot_linux.py currently provides Fedora 31, whose kernel is too
+            old, so this test cannot pass with the on-image kernel (you are
+            welcome to try, hence the option to force such a test with
+            -p vmlinuz='').  Therefore, for now the user must provide a
+            sufficiently new custom kernel, or effectively explicitly
+            request failure with -p vmlinuz=''.
+            Once an image with a sufficiently new kernel is available
+            (probably Fedora 34), we can make -p vmlinuz='' the default, so
+            that this parameter no longer needs to be specified.
+            """
             self.cancel('vmlinuz parameter not set; you must point it to a '
                         'Linux kernel binary to test (to run this test with ' \
                         'the on-image kernel, set it to an empty string)')