kvmtool/x86: implement guest_pre_init

Add the tiny x86/init.S which just mounts /host and execs
/virt/init.

NOTE: of course, the usage of CONFIG_GUEST_PRE_INIT is ugly, we
need to cleanup this code

kvmtool/x86: implement guest_pre_init

Add the tiny x86/init.S which just mounts /host and execs
/virt/init.

NOTE: of course, the usage of CONFIG_GUEST_PRE_INIT is ugly, we
need to cleanup this code. But I'd prefer to do this on top of
this minimal/simple change. And I think this needs cleanups in
any case, for example I think lkvm shouldn't abuse the "init="
kernel parameter at all.

Acked-by: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

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