libvirt: saving the lxc rootfs device in instance metadata

Currently, nbd/loop device, to which lxc image has been
connected, is being saved as instance root device, which causing
the attached bug.
Saving the device name in instance system metadata instead.

Closes-Bug: #1330981
Change-Id: I3a0533a89ea8af61349118a39b822f4af146cc04
Merge "libvirt: saving the lxc rootfs device in instance metadata"