iotests: Make 182 do without device_add

iotests: Make 182 do without device_add

182 fails if qemu has no support for hotplugging of a virtio-blk device.
Using an NBD server instead works just as well for the test, even on
qemus without hotplugging support.

Fixes: 6d0a4a0fb5c8f10c8eb68b52cfda0082b00ae963
Reported-by: Danilo C. L. de Paula <[email protected]>
Signed-off-by: Max Reitz <[email protected]>
Message-Id: <20190417153005[email protected]>
Tested-by: Eric Blake <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Acked-by: Alberto Garcia <[email protected]>
Signed-off-by: Eric Blake <[email protected]>