Add features ext4 targz to target x86/xen_domu in order to
generate images in defconfig.

This fixes #18074.
Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>

Backport of r46233

git-svn-id: svn://svn.openwrt.org/openwrt/branches/chaos_calmer@46277 3c298f89-4303-0410-b956-a3cf2f4a3e73