docs: kvm: convert devices/s390_flic.txt to ReST

- Use standard markup for document title;
- Adjust indentation and add blank lines as needed;
- use the notes markup;
- mark code blocks as such.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>