Don't try to add non-existant devices to ACL

The QEMU driver has a list of devices nodes that are whitelisted
for all guests. The kernel has recently started returning an
error if you try to whitelist a device which does not exist.
This causes a warning in libvirt logs and an audit error for
any missing devices. eg

2013-02-27 16:08:26.515+0000: 29625: warning : virDomainAuditCgroup:451 : success=no virt=kvm resrc=cgroup reason=allow vm="vm031714" uuid=9d8f1de0-44f4-a0b1-7d50-e41ee6cd897b cgroup="/sys/fs/cgroup/devices/libvirt/qemu/vm031714/" class=path path=/dev/kqemu rdev=? acl=rw
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>
(cherry picked from commit 7f544a4c)

src/qemu/qemu_cgroup.c

@@ -263,6 +263,12 @@ int qemuSetupCgroup(struct qemud_driver *driver,
         }
 
         for (i = 0; deviceACL[i] != NULL ; i++) {
+            if (access(deviceACL[i], F_OK) < 0) {
+                VIR_DEBUG("Ignoring non-existant device %s",
+                          deviceACL[i]);
+                continue;
+            }
+
             rc = virCgroupAllowDevicePath(cgroup, deviceACL[i],
                                           VIR_CGROUP_DEVICE_RW);
             virDomainAuditCgroupPath(vm, cgroup, "allow", deviceACL[i], "rw", rc);