LXC from native: import rootfs

LXC rootfs can be either a directory or a block device or an image
file. The first two types have been implemented, but the image file is
still to be done since LXC auto-guesses the file format at mount time
and the LXC driver doesn't support the 'auto' format.

tests/lxcconf2xmldata/lxcconf2xml-simple.xml

@@ -13,5 +13,9 @@
   <on_reboot>restart</on_reboot>
   <on_crash>destroy</on_crash>
   <devices>
+    <filesystem type='mount' accessmode='passthrough'>
+      <source dir='/var/lib/lxc/migrate_test/rootfs'/>
+      <target dir='/'/>
+    </filesystem>
   </devices>
 </domain>