PCI: move pdev_list field to common structure

PCI: move pdev_list field to common structure

Its management shouldn't be arch-specific, and in particular there
should be no need for special precautions when creating the special
domains.

At this occasion
- correct parenthesization of for_each_pdev(),
- stop open-coding for_each_pdev() in vPCI code.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <julien.grall@arm.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>