xen/pci: Allocate memory for physdev_pci_device_add's optarr

physdev_pci_device_add's optarr[] is a zero-sized array and therefore
reference to add.optarr[0] is accessing memory that does not belong to
the 'add' variable.

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