misc/py-anita: Repair xen option and simplify

Restore the xen option and point to xen{kernel,tools}418.  Note that
either 415 or 418 should work.

Stop making qemu be an options group, because it only has one member
and this is likely to continue.  Remove comments about qemu0 and
needing 2.0.0nb4, because that's well more than 5 years in the past.
Pick qemu>=3 as the dependency pattern, to be comfortably past
2.0.0nb4, while not unreasonably requiring last week's qemu.  Likely
newer is needed, but advancing the requirement is left for future
work.

This commit results in no change by default, because all options were
and are default off.  It merely provides better documentation for
emulation/hypervisor choices.

(gdt)