Add support for specifying properties of dynamically-registered devices via the command line

The previous approach of traversing the machine and its device tree
at startup to register CLI11 options was not working for dynamically
registered devices like PCI cards. This meant that options like
gfxmem_size or mon_id from the video cards could not be set.

Switch to instead registering in MachineFactory a hook function that
provides CLI flag values. We can call it when registering any property,
whether at startup or dynamically.

main.cpp

@@ -162,22 +162,31 @@ int main(int argc, char** argv) {
         }
     }
 
-    /* handle overriding of machine settings from command line */
-    map<string, string> settings;
-    if (MachineFactory::get_machine_settings(machine_str, settings) < 0) {
+    // Hook to allow properties to be read from the command-line, regardless
+    // of when they are registered.
+    MachineFactory::get_setting_value = [&](const std::string& name) -> std::optional<std::string> {
+        CLI::App sa;
+        sa.allow_extras();
+
+        std::string value;
+        sa.add_option("--" + name, value);
+        try {
+            sa.parse(app.remaining_for_passthrough());
+        } catch (const CLI::Error& e) {
+            ABORT_F("Cannot parse CLI: %s", e.get_name().c_str());
+        }
+
+        if (sa.count("--" + name) > 0) {
+            return value;
+        } else {
+            return std::nullopt;
+        }
+    };
+
+    if (MachineFactory::register_machine_settings(machine_str) < 0) {
         return 1;
     }
 
-    CLI::App sa;
-    sa.allow_extras();
-
-    for (auto& s : settings) {
-        sa.add_option("--" + s.first, s.second);
-    }
-    sa.parse(app.remaining_for_passthrough()); /* TODO: handle exceptions! */
-
-    MachineFactory::set_machine_settings(settings);
-
     cout << "BootROM path: " << bootrom_path << endl;
     cout << "Execution mode: " << execution_mode << endl;
     if (is_deterministic) {