Fix IMG0..9 config entries in zuluscsi.ini not working (#223)

Commit 06dab18 made scsiDiskGetImageNameFromConfig() use
the configured SCSI ID for lookup, but the ID was only
set after the image was opened.

Made it set the ID in scsiDiskConfigDefaults(),
and then scsiDiskOpenHDDImage() will set S2S_CFG_TARGET_ENABLED.

src/ZuluSCSI_disk.cpp

@@ -452,6 +452,7 @@ static void scsiDiskConfigDefaults(int target_idx)
     preset_config_t defaults = getSystemPreset(presetName);
 
     image_config_t &img = g_DiskImages[target_idx];
+    img.scsiId = target_idx;
     img.deviceType = S2S_CFG_FIXED;
     img.deviceTypeModifier = defaults.deviceTypeModifier;
     img.sectorsPerTrack = defaults.sectorsPerTrack;