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=709471Signed-off-by: NSoren Hansen <soren@linux2go.dk>

src/node_device/node_device_udev.c

@@ -1620,7 +1620,7 @@ static int udevDeviceMonitorStartup(int privileged)
         /* Ignore failure as non-root; udev is not as helpful in that
          * situation, but a non-privileged user won't benefit much
          * from udev in the first place.  */
-        if (privileged || errno != EACCES) {
+        if (errno != ENOENT && (privileged  || errno != EACCES)) {
             char ebuf[256];
             VIR_ERROR(_("Failed to initialize libpciaccess: %s"),
                       virStrerror(pciret, ebuf, sizeof ebuf));