xen/pciback: Do not dereference psdev during printk when it is NULL.

.. instead use BUG_ON() as all the callers of the kill_domain_by_device
check for psdev.

Suggested-by: Jan Beulich <JBeulich@suse.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>