Set sensible defaults for cpu match and feature policy

Set sensible defaults for cpu match and feature policy

To enable the CPU XML from the capabilities to be pasted directly
into the guest XML with no editing, pick a sensible default for
match and feature policy. The CPU match will be exact and the
feature policy will be require. This should ensure safety for
migration and give DWIM semantics for users

* src/conf/cpu_conf.c: Default to exact match and require policy
* docs/formatdomain.html.in: Document new defaults