Don't treat pci_system_init failure as fatal if no PCI bus is present

Don't treat pci_system_init failure as fatal if no PCI bus is present

Xen PV domU's have no PCI bus. node_device_udev.c calls pci_system_init
which looks for /sys/bus/pci. If it does not find /sys/bus/pci (which it
won't in a Xen PV domU) it returns unsuccesfully (ENOENT), which libvirt
considers fatal. This makes libvirt unusable in this environment, even
though there are plenty of valid virtualisation options that work
there (LXC, UML, and QEmu spring to mind)

https://bugzilla.redhat.com/show_bug.cgi?id=709471

Signed-off-by: Soren Hansen <soren@linux2go.dk>