LXC driver: started implementing connectDomainXMLFromNative

This function aims at converting LXC configuration into a libvirt
domain XML descrition to help users migrate from LXC to libvirt.
LXC config2xml: import rootfs
LXC config to xml: migrate fstab and lxc.mount.entry

Tmpfs default usage is computed based on the host physical memory. to
easier test this, the value is computed outside the parse method.
lxc conf from native: implement no network conversion
lxc from native: migrate veth network configuration

Some of the LXC configuration properties aren't migrated since they
would only cause problems in libvirt-lxc:
  lxc.network.ipv[46]: LXC driver doesn't setup IP address of guests
  lxc.network.name