iotests/308: Add test for 'write -zu'

iotests/308: Add test for 'write -zu'

Try writing zeroes to a FUSE export while allowing the area to be
unmapped; block/file-posix.c generally implements writing zeroes with
BDRV_REQ_MAY_UNMAP ('write -zu') by calling fallocate(PUNCH_HOLE).  This
used to lead to a blk_pdiscard() in the FUSE export, which may or may
not lead to the area being zeroed.  HEAD^ fixed this to use
blk_pwrite_zeroes() instead (again with BDRV_REQ_MAY_UNMAP), so verify
that running `qemu-io 'write -zu'` on a FUSE exports always results in
zeroes being written.

Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
Message-Id: <20230227104725.33511-3-hreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>