xen: do not start xen interrupt threads in non-xen guests

If we do, we'll have extra xen threads running around for no reason in the !xen
case. is_xen() should already work here since the initializers are run after
cpuid discovery, which is when the information for is_xen is filled up.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>