softmmu/dirtylimit: Add parameter check for hmp "set_vcpu_dirty_limit"

dirty_rate paraemter of hmp command "set_vcpu_dirty_limit" is invalid
if less than 0, so add parameter check for it.

Note that this patch also delete the unsolicited help message and
clean up the code.

Signed-off-by: Hyman Huang(黄勇) <yong.huang@smartx.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Message-Id: <168618975839.6361.17407633874747688653-1@git.sr.ht>
Signed-off-by: Juan Quintela <quintela@redhat.com>

softmmu/dirtylimit.c

@@ -515,14 +515,15 @@ void hmp_set_vcpu_dirty_limit(Monitor *mon, const QDict *qdict)
     int64_t cpu_index = qdict_get_try_int(qdict, "cpu_index", -1);
     Error *err = NULL;
 
-    qmp_set_vcpu_dirty_limit(!!(cpu_index != -1), cpu_index, dirty_rate, &err);
-    if (err) {
-        hmp_handle_error(mon, err);
-        return;
+    if (dirty_rate < 0) {
+        error_setg(&err, "invalid dirty page limit %" PRId64, dirty_rate);
+        goto out;
     }
 
-    monitor_printf(mon, "[Please use 'info vcpu_dirty_limit' to query "
-                   "dirty limit for virtual CPU]\n");
+    qmp_set_vcpu_dirty_limit(!!(cpu_index != -1), cpu_index, dirty_rate, &err);
+
+out:
+    hmp_handle_error(mon, err);
 }
 
 static struct DirtyLimitInfo *dirtylimit_query_vcpu(int cpu_index)