dump: hoist lzo_init() from get_len_buf_out() to dump_init()

dump: hoist lzo_init() from get_len_buf_out() to dump_init()

qmp_dump_guest_memory()
  dump_init()
    lzo_init() <---------+
  create_kdump_vmcore()  |
    write_dump_pages()   |
      get_len_buf_out()  |
        lzo_init() ------+

This patch doesn't change the fact that lzo_init() is called for every
LZO-compressed dump, but it makes get_len_buf_out() more focused (single
responsibility).

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>