xen-pciback: simplify vpci's find hook

There's no point in comparing SBDF - we can simply compare the struct
pci_dev pointers. If they weren't the same for a given device, we'd have
bigger problems from having stored a stale pointer.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

Link: https://lore.kernel.org/r/158273a2-d1b9-3545-b25d-affca867376c@suse.com
Signed-off-by: Juergen Gross <jgross@suse.com>