qemu: add short document on qemu event handlers

This text was in the commit log for the patch that added the event
handler for NIC_RX_FILTER_CHANGED, and John Ferlan expressed a desire
that the information not be "lost", so I've put it into a file in the
qemu directory, hoping that it might catch the attention of future
writers of handlers for qemu events.

src/qemu/EVENTHANDLERS.txt

+This is a short description of how an example qemu event can be used
+to trigger handler code that is called from the context of a worker
+thread, rather than directly from the event thread (which should
+itself never block, and can't do things like send qemu monitor
+commands, etc).
+
+In this case (the NIC_RX_FILTER_CHANGED event) the event is handled by
+calling a qemu monitor command to get the current RX filter state,
+then executing ioctls/sending netlink messages on the host in response
+to changes in that filter state. This event is *not* propagated to the
+libvirt API (but if someone wants to add details of how to handle that
+to the end of this document, please do!).
+
+Hopefully this narration will be helpful when adding handlers for
+other qemu events in the future.
+
+----------------------------------------------------
+
+Any event emitted by qemu is received by
+qemu_monitor_json.c:qemuMonitorJSONProcessEvent(). It looks up the
+event by name in the table eventHandlers (in the same file), which
+should have an entry like this for each event that libvirt
+understands:
+
+    { "NIC_RX_FILTER_CHANGED", qemuMonitorJSONHandleNicRxFilterChanged, },
+
+  NB: This table is searched with bsearch, so it *must* be
+  alphabetically sorted.
+
+qemuMonitorJSONProcessEvent calls the function listed in
+eventHandlers, e.g.:
+
+   qemu_monitor_json.c:qemuMonitorJSONHandleNicRxFilterChanged()
+
+which extracts any required data from the JSON ("name" in this case),
+and calls:
+
+   qemu_monitor.c:qemuMonitorEmitNicRxFilterChanged()
+
+which uses QEMU_MONITOR_CALLBACK() to call
+mon->cb->domainNicRxFilterChanged(). domainNicRxFilterChanged is one
+in a list of function pointers in qemu_process.c:monitorCallbacks. For
+our example, it has been set to:
+
+   qemuProcessHandleNicRxFilterChanged()
+
+This function allocates a qemuProcessEvent object, and queues an event
+named QEMU_PROCESS_EVENT_NIC_RX_FILTER_CHANGED (you'll want to add an
+enum to qemu_domain.h:qemuProcessEventType for your event) for a
+worker thread to handle.
+
+(Everything up to this point has happened in the context of the thread
+that is reading events from qemu, so it should do as little as
+possible, never block, and never call back into the qemu
+monitor. Everything after this is handled in the context of a worker
+thread, so it has more freedom to make qemu monitor calls and blocking
+system calls on the host.)
+
+When the worker thread gets the event, it calls
+
+   qemuProcessEventHandler()
+
+which switches on the eventType (in our example,
+QEMU_PROCESS_EVENT_NIC_RX_FILTER_CHANGED) and decides to call:
+
+   processNicRxFilterChangedEvent()
+
+and *that* is where the actual work will be done (and any
+event-specific memory allocated during qemuProcessHandleXXX() will be
+freed). Note that this function must do proper refcounting of the
+domain object, and assure that the domain is still active prior to
+performing any operations - it is possible that the domain could have
+been destroyed between the time the event was received and the time
+that it is processed, and it is also possible that the domain could be
+destroyed *during* the event processing if it doesn't get properly
+referenced by the handler.