buildroot

package/xen: Update to version 4.8.1

Update Xen to version 4.8.1 and update the patches to allow Xen
hypervisor and Xen tools to build.

The patches applied were re-aranged a little bit. Patches 2, 3 and 4
have been accepted by upstream Xen and won't be required in future
releases.

Patch 1 (previously patch 2) is still required and was moved to patch 1
to better reflect it's status.

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>