[xs64bit] CP-9457: Read VBD kthread-pid from right path

Before we were reading from:

    /local/domain/0/backend/<vbd|vbd3>/<devid>

When we should also include the domid of the frontend:

    /local/domain/0/backend/<vbd|vbd3>/<domid>/<devid>

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>