Use XDG Base Directories instead of storing in home directory

As defined in:
http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html

This offers a number of advantages:
 * Allows sharing a home directory between different machines, or
sessions (eg. using NFS)
 * Cleanly separates cache, runtime (eg. sockets), or app data from
user settings
 * Supports performing smart or selective migration of settings
between different OS versions
 * Supports reseting settings without breaking things
 * Makes it possible to clear cache data to make room when the disk
is filling up
 * Allows us to write a robust and efficient backup solution
 * Allows an admin flexibility to change where data and settings are stored
 * Dramatically reduces the complexity and incoherence of the
system for administrators

docs/auth.html.in

@@ -25,7 +25,7 @@ for the authentication file using the following sequence:
         variable.</li>
       <li>The file path specified by the "authfile=/some/file" URI
         query parameter</li>
-      <li>The file $HOME/.libvirt/auth.conf</li>
+      <li>The file $XDG_CONFIG_DIR/libvirt/auth.conf</li>
       <li>The file /etc/libvirt/auth.conf</li>
     </ol>
 

docs/uri.html.in

@@ -30,7 +30,7 @@ virConnectPtr conn = virConnectOpenReadOnly (<b>"test:///default"</b>);
     <p>
 To simplify life for administrators, it is possible to setup URI aliases in a
 libvirt client configuration file. The configuration file is <code>/etc/libvirt/libvirt.conf</code>
-for the root user, or <code>$HOME/.libvirt/libvirt.conf</code> for any unprivileged user.
+for the root user, or <code>$XDG_CONFIG_DIR/libvirt/libvirt.conf</code> for any unprivileged user.
 In this file, the following syntax can be used to setup aliases
     </p>