kvm tools: fix zombie reaping in guest/init.c

init.c is not very kind with processes that get reparented when
their own parent die, leaving them hanging around. Looking at the
code, it only seem to care about its own flesh and blood. Bad init.

Teach it some basic zombie reaping skills.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>