conf,qemu: Check for NULL addrs in virDomainUSBAddressRelease
    
    Rather than having the caller check, if the input @addrs is NULL
    (e.g. priv->usbaddrs), then just return 0. This also removes the
    need for ATTRIBUTE_NONNULL which only really helped if someone
    passed a NULL as a parameter not if the passed parameter is NULL.
    
    Signed-off-by: John Ferlan <jferlan@redhat.com>