docs: virt: guest-halt-polling.txt convert to ReST

Due to some merge conflict, this file ended being alone under
Documentation/virtual.

The file itself is almost at ReST format. Just minor
adjustments are needed:

- Adjust title markup;
- Adjust a list identation;
- add a literal block markup;
- Add some blank lines.

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