Host folder sharing via USB issue

[Programmingkid <programmingkidx@gmail.com>]

Hi I have noticed that host folder sharing via USB has recently stopped working. After doing some git bisecting I found this as the patch that seems to be the issue:

25f78d9e2de528473d52acfcf7acdfb64e3453d4 is the first bad commit
commit 25f78d9e2de528473d52acfcf7acdfb64e3453d4
Author: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Date:   Thu Jun 10 15:05:34 2021 +0300

    block: move supports_backing check to bdrv_set_file_or_backing_noperm()
    
    Move supports_backing check of bdrv_reopen_parse_backing to called
    (through bdrv_set_backing_noperm()) bdrv_set_file_or_backing_noperm()
    function. The check applies to general case, so it's appropriate for
    bdrv_set_file_or_backing_noperm().
    
    We have to declare backing support for two test drivers, otherwise new
    check fails.
    
    Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
    Message-Id: <20210610120537.196183-7-vsementsov@virtuozzo.com>
    Signed-off-by: Kevin Wolf <kwolf@redhat.com>

 block.c                          | 29 +++++++++++++++--------------
 tests/unit/test-bdrv-drain.c     |  1 +
 tests/unit/test-bdrv-graph-mod.c |  1 +
 3 files changed, 17 insertions(+), 14 deletions(-)

To reproduce this issue run this command:
qemu-system-i386 -usb -device usb-storage,drive=fat16 -drive file=fat:rw:fat-type=16:"<path to host folder>",id=fat16,format=raw,if=none

Results:
Unexpected error in bdrv_set_file_or_backing_noperm() at ../block.c:3159:
qemu-system-i386: -drive file=fat:rw:fat-type=16:<host folder path>,id=fat16,format=raw,if=none: Driver 'vvfat' of node '#block057' does not support backing files
Abort trap: 6

Expected results:
QEMU start running normally.

Please let me know if you need more information.

Thank you.