scsi, pci, qdev, isa-bus, sysbus: don't let *_get_fw_dev_path return NULL

Use g_strdup rather than strdup, because the sole caller
(qdev_get_fw_dev_path_helper) assumes it gets non-NULL, and dereferences
it.  Besides, in that caller, the allocated buffer is already freed with
g_free, so it's better to allocate with a matching g_strdup.

In one case, (scsi-bus.c) it was trivial, so I replaced an snprintf+
g_strdup combination with an equivalent g_strdup_printf use.

Signed-off-by: Jim Meyering <meyering@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/hw/ide/qdev.c b/hw/ide/qdev.c
index 5ea9b8f..f2e4ea4 100644
--- a/hw/ide/qdev.c
+++ b/hw/ide/qdev.c
@@ -60,7 +60,7 @@
     snprintf(path, sizeof(path), "%s@%d", qdev_fw_name(dev),
              ((IDEBus*)dev->parent_bus)->bus_id);
 
-    return strdup(path);
+    return g_strdup(path);
 }
 
 static int ide_qdev_init(DeviceState *qdev)