Much like for gnttab and foreignmemory xencall hypercall buffers need
care.
Evtchn is a bit simpler (no magic mappings) but may not work from
parent + child simultaneously, document "parent only" since it is
consistent with the others.
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>