xen/virtio: Fix NULL deref when a bridge of PCI root bus has no parent

When attempting to run Xen on a QEMU/KVM virtual machine with virtio
devices (all x86_64), function xen_dt_get_node() crashes on accessing
bus->bridge->parent->of_node because a bridge of the PCI root bus has no
parent set:

[    1.694192][    T1] BUG: kernel NULL pointer dereference, address: 0000000000000288
[    1.695688][    T1] #PF: supervisor read access in kernel mode
[    1.696297][    T1] #PF: error_code(0x0000) - not-present page
[    1.696297][    T1] PGD 0 P4D 0
[    1.696297][    T1] Oops: 0000 [#1] PREEMPT SMP NOPTI
[    1.696297][    T1] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 6.3.7-1-default #1 openSUSE Tumbleweed a577eae57964bb7e83477b5a5645a1781df990f0
[    1.696297][    T1] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.15.0-0-g2dd4b9b-rebuilt.opensuse.org 04/01/2014
[    1.696297][    T1] RIP: e030:xen_virtio_restricted_mem_acc+0xd9/0x1c0
[    1.696297][    T1] Code: 45 0c 83 e8 c9 a3 ea ff 31 c0 e...