PCI host bridges need to set fw_name to be discoverable
by bios for boot device selection.
In particular, seabios expects root device to be called
"/pci/@i0cf8", so let's set it up like that for Q35.
Cc: qemu-stable@nongnu.org
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Amos Kong <akong@redhat.com>