qapi script: add event support

qapi-event.py will parse the schema and generate qapi-event.c, then
the API in qapi-event.c can be used to handle events in qemu code.
All API have prefix "qapi_event".

The script mainly includes two parts: generate API for each event
define, generate an enum type for all defined events.

Since in some cases the real emit behavior may change, for example,
qemu-img would not send a event, a callback layer is used to
control the behavior. As a result, the stubs at compile time
can be saved, the binding of block layer code and monitor code
will become looser.

Signed-off-by: Wenchao Xia <wenchaoqemu@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>

tests/qapi-schema/event-nest-struct.err

@@ -0,0 +1 @@
+tests/qapi-schema/event-nest-struct.json:1: Nested structure define in event is not supported now, event 'EVENT_A', argname 'a'

tests/qapi-schema/event-nest-struct.exit

@@ -0,0 +1 @@
+1

tests/qapi-schema/event-nest-struct.json

@@ -0,0 +1,2 @@
+{ 'event': 'EVENT_A',
+  'data': { 'a' : { 'string' : 'str', 'integer': 'int' }, 'b' : 'str' } }