xen/pci: Make use of the helper macro LIST_HEAD()

Replace "struct list_head head = LIST_HEAD_INIT(head)" with
"LIST_HEAD(head)" to simplify the code.

Signed-off-by: Cai Huoqing <cai.huoqing@linux.dev>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20220209032842.38818-1-cai.huoqing@linux.dev


Signed-off-by: Juergen Gross <jgross@suse.com>