I continue my tests.
I've found another problem : when I restart the host server, domain autostart fails occasionally. Here is the libvirtd.log when the problem appears:
2012-01-20 13:40:10.165+0000: 4674: info : libvirt version: 0.9.8
2012-01-20 13:40:10.165+0000: 4674: error : virCommandWait:2192 : internal error Child process (/bin/sh -c EBT=/sbin/ebtables
cmd='$EBT -t nat -L'
eval res=\$\("${cmd} 2>&1"\)
if [ $? -ne 0 ]; then echo "Failure to execute command '${cmd}' : '${res}'."; exit 1;fi
) status unexpected: exit status 1
2012-01-20 13:40:11.990+0000: 4659: error : qemuMonitorIORead:470 : Unable to read from monitor: Connection reset by peer
2012-01-20 13:40:11.990+0000: 4674: error : qemuMonitorTextGetPtyPaths:2355 : operation failed: failed to retrieve chardev info in qemu with 'info chardev'
2012-01-20 13:40:12.190+0000: 4674: error : qemuAutostartDomain:168 : Failed to autostart VM 'vm001': operation failed: failed to retrieve chardev info in qemu with 'info chardev'