virsh: Add support for modifying domain description and titles

virsh: Add support for modifying domain description and titles

This patch adds a new command "desc" to show and modify titles and
description for the domains using the new API.

This patch also adds a new flag for the "list" command to show titles in
the domain list, to allow easy identification of VMs by storing a short
description.

Example:
virsh # list --title
 Id Name                 State      Title
 -----------------------------------------------
   0 Domain-0             running    Mailserver 1
   2 fedora               paused