[libvirt] Add <console> support

[Christophe Fergeau]

Hi,

This patch series adds support for setting <console> nodes as has been
discussed in the "[PATCH libvirt-glib 02/15] Add objects for dealing with
<console> devices" thread.

It introduces a GVirConfigDomainChardev which derives from
GVirConfigDomainDevice. The actual classes to create <console>, <serial>,
<parallel> nodes will inherit from this parent class.

Then we have GVirConfigDomainChardevSource which derives from
GVirConfigObject (with a dummy root node name) and from which derives
GVirConfigDomainChardevSourcePty.

A GVirConfigDomainChardevSourcePty is associated to a
GVirConfigDomainConsole object through GVirConfigDomainChardev::add_source.
Maybe it should be ::set_source, I'm unsure... The code from ::add_source
directly messes with the xml nodes so a careful review won't hurt :)

At this point I consider this to be at least 50% RFC so we can still
redesign the whole thing.

Christophe