vmbus: reserved channels are not saved when the guest is disconnected

[SvenGroot]

Reserved channels can be open even if the guest is not currently connected to host vmbus. However, vmbus_server only saves its channel list if the guest is connected. This can cause reserved channels to be lost.

The chance of this is small, as the only realistic window this can happen is between the UEFI client disconnecting and the main guest OS connecting, and reserved channels are rare (only used by kdnet).