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
lxc from native: convert phys network types to net hostdev devices
lxc from native: convert lxc.tty to console devices
lxc from native: convert macvlan network configuration
LXC from native: convert lxc.id_map into <idmap>
LXC from native: migrate memory tuning
LXC from native: map lxc.cgroup.cpu.*
LXC from native: map lxc.cgroup.cpuset.*
LXC from native: add lxc.cgroup.blkio.* mapping