Description of problem:
If manually edit LXC guest XML with line <seclabel type='none'/> and without security model, the libvirtd will crash when to start the LXC guest.

Version-Release number of selected component (if applicable):
# rpm -q libvirt kernel
libvirt-0.10.2-21.el6.x86_64
kernel-2.6.32-288.el6.x86_64


How reproducible:
always

Steps to Reproduce:
1. virsh edit "<seclabel type='none'/>" line into LXC guest XML and save configuration
2. virsh start <domain>


Actual results:

error: Failed to start domain toy
error: End of file while reading data: Input/output error

Expected results:
no crash

Additional info:

# virsh -c lxc:/// dumpxml toy
<domain type='lxc'>
  <name>toy</name>
  <uuid>bb428983-cb9f-4702-0f8d-7d4e143d9aad</uuid>
  <memory unit='KiB'>500000</memory>
  <currentMemory unit='KiB'>500000</currentMemory>
  <vcpu placement='static'>4</vcpu>
  <os>
    <type arch='x86_64'>exe</type>
    <init>/bin/sh</init>
  </os>
  <clock offset='utc'/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>destroy</on_crash>
  <devices>
    <emulator>/usr/libexec/libvirt_lxc</emulator>
    <filesystem type='mount' accessmode='passthrough'>
      <source dir='/'/>
      <target dir='/'/>
    </filesystem>
    <console type='pty'>
      <target type='lxc' port='0'/>
    </console>
  </devices>
  <seclabel type='none'/>
</domain>


# virsh -c lxc:/// start toy
error: Failed to start domain toy
error: End of file while reading data: Input/output error

# service libvirtd status
libvirtd dead but pid file exists


Notes, it's okay for QEMU driver, the "model='selinux'" will be automatically append into line "<seclabel type='none'/>", so we probably need to add security driver for label selinux with LXC driver. In addition, operation virDomainCreate forbidden for read only access, but not sure whether other callers also use 'virSecurityManagerGenLabel' via a read-only client then crash libvirtd.


GDB backtrace:

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7f638599b700 (LWP 24630)]
__strcmp_ssse3 () at ../sysdeps/x86_64/strcmp.S:2112
2112            movdqa  (%rdi), %xmm2
(gdb) bt
#0  __strcmp_ssse3 () at ../sysdeps/x86_64/strcmp.S:2112
#1  0x0000003655d99299 in virSecurityManagerGenLabel (mgr=<value optimized out>, vm=0x7f637c2f7280) at security/security_manager.c:337
#2  0x00000000004d210d in virLXCProcessStart (conn=0x7f636c000ae0, driver=0x7f637c22e280, vm=0x7f637c2bfcb0, autoDestroy=false, reason=VIR_DOMAIN_RUNNING_BOOTED) at lxc/lxc_process.c:996
#3  0x00000000004cd4ed in lxcDomainStartWithFlags (dom=0x7f6364000900, flags=0) at lxc/lxc_driver.c:1007
#4  0x0000003655cf56f0 in virDomainCreate (domain=0x7f6364000900) at libvirt.c:8319
#5  0x0000000000440212 in remoteDispatchDomainCreate (server=<value optimized out>, client=<value optimized out>, msg=<value optimized out>, rerr=0x7f638599ab80, args=<value optimized out>, 
    ret=<value optimized out>) at remote_dispatch.h:1066
#6  remoteDispatchDomainCreateHelper (server=<value optimized out>, client=<value optimized out>, msg=<value optimized out>, rerr=0x7f638599ab80, args=<value optimized out>, ret=<value optimized out>)
    at remote_dispatch.h:1044
#7  0x0000003655d401e2 in virNetServerProgramDispatchCall (prog=0xcff940, server=0xcf6ef0, client=0xcfdc90, msg=0xcfc4b0) at rpc/virnetserverprogram.c:431
#8  virNetServerProgramDispatch (prog=0xcff940, server=0xcf6ef0, client=0xcfdc90, msg=0xcfc4b0) at rpc/virnetserverprogram.c:304
#9  0x0000003655d414ce in virNetServerProcessMsg (srv=<value optimized out>, client=0xcfdc90, prog=<value optimized out>, msg=0xcfc4b0) at rpc/virnetserver.c:170
#10 0x0000003655d41b6c in virNetServerHandleJob (jobOpaque=<value optimized out>, opaque=0xcf6ef0) at rpc/virnetserver.c:191
#11 0x0000003655c63e9c in virThreadPoolWorker (opaque=<value optimized out>) at util/threadpool.c:144
#12 0x0000003655c63789 in virThreadHelper (data=<value optimized out>) at util/threads-pthread.c:161
#13 0x00000038baa077f1 in start_thread (arg=0x7f638599b700) at pthread_create.c:301
#14 0x00000033f68e570d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:115