libvirt: fill in metadata when launching instances

When generating the guest XML config, fill in the nova
specific metadata whose schema was previously defined.

Blueprint: libvirt-driver-domain-metadata
Change-Id: I3e1f0eb5bb0e71732a18609ff11f8ee6deb1a8da
Change LOG.warn to LOG.debug in _shutdown_instance

Change LOG.warn to LOG.debug for detaching volume related exceptions
in _shutdown_instance in compute/manager.py, since as the log message
said the exceptions should be ignored. It's probably not worth a
warning in the log.

Change-Id: Ibe216b9bccf590e64aa1b966753bd1ec67c8dbdf
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: fill in metadata when launching instances"
Merge "Change LOG.warn to LOG.debug in _shutdown_instance"
Merge "libvirt: saving the lxc rootfs device in instance metadata"