Remove nomodeset for all release images for consistency
We have serial console enabled so we don't need to be able to run qemu
with -curses, which needed nomodeset.
Its quite helpful to have this on the xen iso and it was there until:
bcdaeacd52 main/xen: Remove dependency on syslinux
xen does not depend on syslinux so that is correct, but it had the
effect of syslinux also missing from the iso, and that breaks
setup-bootable
standard iso is now vanilla kernel so it does not make any sense to have
a separata vanilla iso.
Also update xen to use vanilla kernel instead of hardened
