Install virtualbox-ose 4.x guest packages from Debian backports.

Hence remove our home-made backports.
Rationale: those are compatible with the 2.6.38+ kernel we will probably ship
soon.

config/chroot_apt/preferences

@@ -14,6 +14,22 @@ Package: macchanger
 Pin: release o=Debian,a=unstable
 Pin-Priority: 999
 
+Package: virtualbox-ose-guest-dkms
+Pin: origin backports.debian.org
+Pin-Priority: 999
+
+Package: virtualbox-ose-guest-source
+Pin: origin backports.debian.org
+Pin-Priority: 999
+
+Package: virtualbox-ose-guest-utils
+Pin: origin backports.debian.org
+Pin-Priority: 999
+
+Package: virtualbox-ose-guest-x11
+Pin: origin backports.debian.org
+Pin-Priority: 999
+
 Explanation: weirdness in chroot_apt install-binary
 Package: *
 Pin: release o=chroot_local-packages