Install the Spice agent from Debian sid.

This is one of the first bricks needed to properly support running in a Spice
environment, with QXL graphics.

config/chroot_apt/preferences

@@ -238,6 +238,10 @@ Package: laptop-mode-tools
 Pin: release o=Debian,a=unstable
 Pin-Priority: 999
 
+Package: spice-vdagent
+Pin: release o=Debian,a=unstable
+Pin-Priority: 999
+
 Package: xul-ext-adblock-plus
 Pin: release o=Debian,a=unstable
 Pin-Priority: 999

config/chroot_local-packageslists/tails-common.list

@@ -289,3 +289,6 @@ python-urlgrabber
 gnome-theme-xpluna
 gnome-xp-icon-theme
 xp-cursor-theme
+
+### Spice and QXL
+spice-vdagent