[libvirt] Add <console> support

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