hw/pci-bridge: Fix invalid free()

hw/pci-bridge: Fix invalid free()

When loadvm'ing a *running* snapshot qemu crashes due to an invalid
free. It's fortunately caught early by glibc heap memory corruption
protection and qemu gets killed with SIGABRT.

Steps to reproduce:

1) Create VM (e.g w/ virsh define)
2) Start the VM and take a snapshot while it's running and having a
   PCI bridge attached
3) Destroy the VM and revert the running snapshot.

This commit fixes the issue.

Signed-off-by: Matthias Weckbecker <matthias@weckbecker.name>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>