Now that struct vm and struct vcpu are defined in headers, provide
inline accessors. We could just remove the accessors outright, but they
don't hurt and it would result in unneeded churn.
As a part of this, consolidate definitions related to struct
vm_eventinfo as well. I'm not sure if struct vm_eventinfo is really
needed now that vmmops_run implementations can directly access vm and
vcpu fields.
No functional change intended.