constify drive_get_by_id arg

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Message-Id: 
diff --git a/vl.c b/vl.c
index 6c4a509..d82c3d8 100644
--- a/vl.c
+++ b/vl.c
@@ -1835,7 +1835,7 @@
     return NULL;
 }
 
-DriveInfo *drive_get_by_id(char *id)
+DriveInfo *drive_get_by_id(const char *id)
 {
     DriveInfo *dinfo;