plugin/loader: handle basic help query

As the list of options isn't fixed we do all the parsing by hand.
Without any named arguments we automatically fill the "file" option
with the value give so check if it is requesting help and dump some
basic usage text.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240729144414.830369-15-alex.bennee@linaro.org>

plugins/loader.c

@@ -18,6 +18,7 @@
 #include "qemu/osdep.h"
 #include "qemu/error-report.h"
 #include "qemu/config-file.h"
+#include "qemu/help_option.h"
 #include "qapi/error.h"
 #include "qemu/lockable.h"
 #include "qemu/option.h"
@@ -98,7 +99,12 @@ static int plugin_add(void *opaque, const char *name, const char *value,
     bool is_on;
     char *fullarg;
 
-    if (strcmp(name, "file") == 0) {
+    if (is_help_option(value)) {
+        printf("Plugin options\n");
+        printf("  file=<path/to/plugin.so>\n");
+        printf("  plugin specific arguments\n");
+        exit(0);
+    } else if (strcmp(name, "file") == 0) {
         if (strcmp(value, "") == 0) {
             error_setg(errp, "requires a non-empty argument");
             return 1;