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 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.
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
LXC from native: map lxc.arch to /domain/os/type@arch
LXC from native: map block filesystems
LXC from native: map vlan network type

The problem with VLAN is that the user still has to manually create the
vlan interface on the host. Then the generated configuration will use
it as a nerwork hostdev device. So the generated configurations of the
following two fragments are equivalent.

lxc.network.type = phys
lxc.network.link = eth0.5

lxc.network.type = vlan
lxc.network.link = eth0
lxc.network.vlan.id = 5