fix #4435: devices list: avoid error for undefined value

fix #4435: devices list: avoid error for undefined value

When $d->{'pci_bridge'}->{devices} is undef, @-dereferencing it will
die with:
> Can't use an undefined value as an ARRAY reference

This can happen (at least) when the VM is in 'prelaunch' state. The
QAPI definition for '@PciBridgeInfo' also declares the 'devices'
member as optional.

Before commit 721624b ("collect device list for nested pci-bridges"),
there was no issue, because $d->{'pci_bridge'}->{devices} was used in
foreach, so auto-vivified if undef.

Fixes: f721624b ("collect device list for nested pci-bridges")
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>