qmp: Move dispatcher to a coroutine

This moves the QMP dispatcher to a coroutine and runs all QMP command
handlers that declare 'coroutine': true in coroutine context so they
can avoid blocking the main loop while doing I/O or waiting for other
events.

For commands that are not declared safe to run in a coroutine, the
dispatcher drops out of coroutine context by calling the QMP command
handler from a bottom half.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20201005155855.256490-10-kwolf@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

qapi/qmp-registry.c

@@ -20,6 +20,9 @@ void qmp_register_command(QmpCommandList *cmds, const char *name,
 {
     QmpCommand *cmd = g_malloc0(sizeof(*cmd));
 
+    /* QCO_COROUTINE and QCO_ALLOW_OOB are incompatible for now */
+    assert(!((options & QCO_COROUTINE) && (options & QCO_ALLOW_OOB)));
+
     cmd->name = name;
     cmd->fn = fn;
     cmd->enabled = true;