Patch from Ian Campbell.
i386 Xen guests require a PAE (686-bigmem) kernel in order to
run. Therefore this variant includes the relevant installer kernel and
ramdisk in install.386/xen as well as suitable kernel udebs and proper
debs for the installed system.
amd64 Xen has no similar requirement but we include the kernels under
install.amd/xen in order to have a consistent path under both
architectures.