libvirt: lxc: don't mkdir when selinux is disabled

libvirt: lxc: don't mkdir when selinux is disabled

libvirt lxc will fail to start when selinux is disabled.
error: Failed to start domain noroot
error: internal error guest failed to start: PATH=/bin:/sbin TERM=linux container=lxc-libvirt container_uuid=b9873916-3516-c199-8112-1592ff694a9e LIBVIRT_LXC_UUID=b9873916-3516-c199-8112-1592ff694a9e LIBVIRT_LXC_NAME=noroot /bin/sh
2013-01-09 11:04:05.384+0000: 1: info : libvirt version: 1.0.1
2013-01-09 11:04:05.384+0000: 1: error : lxcContainerMountBasicFS:546 : Failed to mkdir /sys/fs/selinux: No such file or directory
2013-01-09 11:04:05.384+0000: 7536: info : libvirt version: 1.0.1
2013-01-09 11:04:05.384+0000: 7536: error : virLXCControllerRun:1466 : error receiving signal from container: Input/output error
2013-01-09 11:04:05.404+0000: 7536: error : virCommandWait:2287 : internal error Child process (ip link del veth1) unexpected exit status 1: Cannot find device "veth1"

fix this problem by checking if selinuxfs is mounted
in host before we try to create dir /sys/fs/selinux.

Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>