Comment 2 for bug 1684239

The mentioned commit is:

commit ebb72c9f066e5f85259e1541a6d3fb5bfd6e73ff
Author: Kevin Wolf <email address hidden>
Date: Wed Apr 27 14:11:38 2016 +0200

    vvfat: Fix volume name assertion

    Commit d5941dd made the volume name configurable, but it didn't consider
    that the rw code compares the volume name string to assert that the
    first directory entry is the volume name. This made vvfat crash in rw
    mode.

    This fixes the assertion to compare with the configured volume name
    instead of a literal string.

    Cc: <email address hidden>
    Signed-off-by: Kevin Wolf <email address hidden>
    Reviewed-by: Markus Armbruster <email address hidden>
    Reviewed-by: Stefan Hajnoczi <email address hidden>

The fix is present since v2.6.0
As you're using v2.5.0, can you try with a newer QEMU version?