Xen events: prevent initialization for PV domains and perform more sanity checks

Provides better error messaging when attempting to initialize events for PV domains, addressing http://code.google.com/p/vmitools/issues/detail?id=48 

and addresses NULL dereferences in http://code.google.com/p/vmitools/issues/detail?id=48 (this current PR provides some insurance with more careful validity checks, though the root problem of issue 48 was solved by PR#5 )