add header guards for non-trivial headers

Add header guards to headers that do not simply include another one.

Signed-off-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210610135937.94375-3-cohuck@redhat.com>