iotests: add test for stream job with an unaligned prefetch read

iotests: add test for stream job with an unaligned prefetch read

Previously, bdrv_pad_request() could not deal with a NULL qiov when
a read needed to be aligned. During prefetch, a stream job will pass a
NULL qiov. Add a test case to cover this scenario.

By accident, also covers a previous race during shutdown, where block
graph changes during iteration in bdrv_flush_all() could lead to
unreferencing the wrong block driver state and an assertion failure
later.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Message-ID: <20240322095009.346989-5-f.ebner@proxmox.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit 12d7b3bbd3333cededd3b695501d8d247239d769)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>