qa: fix test_rbd_wnbd.py

qa: fix test_rbd_wnbd.py

The rbd-wnbd Python test now fails as the wnbd driver changed
the bus type from "virtual" to "SAS" in order to accommodate
Microsoft Failover Cluster.

SAS is considered a shared bus, so with the default SAN policy
(offlineShared), the disks will be offline/read-only by
default[1][2].

That being considered, we're updating the test to set the rw disk
flag before attempting any write operations.

[1] https://learn.microsoft.com/en-us/windows-hardware/customize/desktop/unattend/microsoft-windows-partitionmanager-sanpolicy
[2] https://learn.microsoft.com/en-us/windows-server/administration/windows-commands/san

Signed-off-by: Lucian Petrut <lpetrut@cloudbasesolutions.com>