kvm tools: Merge guest init and init_stage2.c

kvm tools: Merge guest init and init_stage2.c

Two stages init process is introduced in

   commit eaf720b285947a6f4e29174d0eba1899de31d8ab
   kvm tools: Split custom rootfs init into two stages

to address the outdated init binary issue.

Currenly, init binary is embedded in lkvm binary and generated on the
fly. So there is no need to split the init process.

This makes
   1) the size of lkvm binary smaller becasue only one
      statically linked init binary.
   2) the init process simpler and cleaner.

Signed-off-by: Asias He <asias.hejun@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>