In GCE, the pci bus looks like:

   $ lspci -x
   00:01.0 ISA bridge: Intel Corporation 82371AB/EB/MB PIIX4 ISA (rev 03)
   00: 86 80 10 71 03 01 00 00 03 00 01 06 00 00 80 00
   10: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
   20: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
   30: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

   00:01.3 Bridge: Intel Corporation 82371AB/EB/MB PIIX4 ACPI (rev 03)
   00: 86 80 13 71 03 01 00 00 03 00 80 06 00 00 00 00
   10: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
   20: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
   30: 00 00 00 00 00 00 00 00 00 00 00 00 09 01 00 00

   00:03.0 SCSI storage controller: Red Hat, Inc Virtio SCSI
   00: f4 1a 04 10 03 01 00 00 00 00 00 01 00 00 00 00
   10: 01 c0 00 00 00 00 00 00 00 00 00 00 00 00 00 00
   20: 00 00 00 00 00 00 00 00 00 00 00 00 f4 1a 08 00
   30: 00 00 00 00 00 00 00 00 00 00 00 00 0b 01 00 00

   00:04.0 Ethernet controller: Red Hat, Inc Virtio network device
   00: f4 1a 00 10 03 01 00 00 00 00 00 02 00 00 00 00
   10: 41 c0 00 00 00 00 00 00 00 00 00 00 00 00 00 00
   20: 00 00 00 00 00 00 00 00 00 00 00 00 f4 1a 01 00
   30: 00 00 00 00 00 00 00 00 00 00 00 00 0b 01 00 00

This makes the code goes to !single_bus branch. However, device 00:00:0
is not present. So, No device is scanned! Obviously, this is not what
we want.

To fix, the following is almost correct except bus 0 might not be there.

   void pci_device_enumeration(void)
   {
          check_bus(0);
   }

To address it, we scan starting from bus 0 and exit the scan when the
first non-empty bus is found. The first non-empty bus should find the
entire hierarchy.

Signed-off-by: Asias He <asias@cloudius-systems.com>