iotests: Add test for reset/AioContext switches with NBD exports

iotests: Add test for reset/AioContext switches with NBD exports

This replicates the scenario in which the bug was reported.
Unfortunately this relies on actually executing a guest (so that the
firmware initialises the virtio-blk device and moves it to its
configured iothread), so this can't make use of the qtest accelerator
like most other test cases. I tried to find a different easy way to
trigger the bug, but couldn't find one.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20240314165825.40261-3-kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit e8fce34eccf68a32f4ecf2c6f121ff2ac383d6bf)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>