xen/pt: Make xen_pt_unregister_device idempotent

xen/pt: Make xen_pt_unregister_device idempotent

To deal with xen_host_pci_[set|get]_ functions returning error values
and clearing ourselves in the init function we should make the
.exit (xen_pt_unregister_device) function be idempotent in case
the generic code starts calling .exit (or for fun does it before
calling .init!).

Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>