Add soft device removal using PnP

[petrutlucian94]

Add soft device removal using PnP

We'll update libwnbd to allow notifying the PnP stack before asking the driver to disconnect a device. PnP subscribers (e.g. filesystem or volume managers) can veto device removal requests until all data has been flushed.

We'll add a configurable timeout and a hard removal fallback option. Since we're still in beta phase, we're going to go ahead and change the device remove API.

On the driver side, we're updating the PnP capabilities. Disabling "SurpriseRemovalOk" says that the device is stateful (we're holding userspace connections) and the driver must be notified when a device is removed. At the same time, we're enabling EjectSupported.

We're adding a "show" command, making it easier for libwnbd to retrieve the disk serial number based on the instance name, which is then needed when iterating over PnP device lists.

Ideally, our driver should react to PnP device removal events. TL;DR: at the moment it doesn't, so we'll need an additional IOCTL_WNBD_UNMAP request. The long story is that we can receive PnP remove events either through IRP_MN_REMOVE_DEVICE or StorRemoveDevice. The latter would be preferred because it should provide us the device extension reference as well as the SCSI disk address. Unfortunately, we're not receiving those, just getting IRP_MN_REMOVE_DEVICE, which only provides the PDO, which seems to have the wrong extension address.

To make it easier to handle PnP removal events, we're moving the synchronous device removal logic to WnbdDisconnectSync, which will be the counterpart of the existing WnbdDisconnectAsync.

Depends-On: https://github.com/cloudbase/wnbd/pull/22

[aserdean]

Thanks a lot for all the work, testing looks good!

Thank you for addressing the comments and including the fixes!