CODING_STYLE: Document how to handle unexpected conditions

CODING_STYLE: Document how to handle unexpected conditions

It's not always clear what the best way is to handle unexpected
conditions: whether with ASSERT(), domain_crash(), BUG_ON(), or some
other method.  All methods have a risk of introducing security
vulnerabilities and unnecessary instabilities to production systems.

Provide guidelines for different options and when to use them.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <julien@xen.org>