hw/i386/pc: Add missing property descriptions

When running "qemu-system-x86_64 -M pc,help" I noticed that some
properties were still missing their description. Add them now so
that users get at least a slightly better idea what they are all
about.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20211206134255.94784-1-thuth@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/i386/pc.c

@@ -1759,15 +1759,23 @@ static void pc_machine_class_init(ObjectClass *oc, void *data)
 
     object_class_property_add_bool(oc, PC_MACHINE_SMBUS,
         pc_machine_get_smbus, pc_machine_set_smbus);
+    object_class_property_set_description(oc, PC_MACHINE_SMBUS,
+        "Enable/disable system management bus");
 
     object_class_property_add_bool(oc, PC_MACHINE_SATA,
         pc_machine_get_sata, pc_machine_set_sata);
+    object_class_property_set_description(oc, PC_MACHINE_SATA,
+        "Enable/disable Serial ATA bus");
 
     object_class_property_add_bool(oc, PC_MACHINE_PIT,
         pc_machine_get_pit, pc_machine_set_pit);
+    object_class_property_set_description(oc, PC_MACHINE_PIT,
+        "Enable/disable Intel 8254 programmable interval timer emulation");
 
     object_class_property_add_bool(oc, "hpet",
         pc_machine_get_hpet, pc_machine_set_hpet);
+    object_class_property_set_description(oc, "hpet",
+        "Enable/disable high precision event timer emulation");
 
     object_class_property_add_bool(oc, "default-bus-bypass-iommu",
         pc_machine_get_default_bus_bypass_iommu,