fdc: make optional

Ignore failure with fdc device creation.
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

hw/fdc.h

@@ -11,7 +11,10 @@ static inline void fdctrl_init_isa(DriveInfo **fds)
 {
     ISADevice *dev;
 
-    dev = isa_create("isa-fdc");
+    dev = isa_try_create("isa-fdc");
+    if (!dev) {
+        return;
+    }
     if (fds[0]) {
         qdev_prop_set_drive_nofail(&dev->qdev, "driveA", fds[0]->bdrv);
     }