Re-use local session libvirt connection

This change also introduces the concept of 'default_connection' which by
default means 'qemu:///session' but could be easily configured to use
some other libvirt connection by modifying the "QEMU Session" file
(hence the reason why connection hashtable now uses name rather than
uris as keys).

https://bugzilla.gnome.org/show_bug.cgi?id=666366