hw/pvrdma: Unregister from shutdown notifier when device goes down

hw/pvrdma: Unregister from shutdown notifier when device goes down

This hook was installed to close the device when VM is going down.
After the device is closed there is no need to be informed on VM
shutdown.

Signed-off-by: Yuval Shaia <yuval.shaia@oracle.com>
Reviewed-by: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Message-Id: <1552300155-25216-11-git-send-email-yuval.shaia@oracle.com>
Reviewed-by: Kamal Heib <kamalheib1@gmail.com>
Signed-off-by: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>