README: Fix markdown formatting

There are formatting issues that prevent README.md
from being rendered correctly in a browser. This
patch fixes the following categories of issues:

- blocks which aren't indented correctly;
- texts wrapped in <> which need escape, or
  be replaced with another thing.

Also some inline commands are marked with ``.

Signed-off-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>