crosvm: re-add '-d' short arg

Re-add the '--disk' short arg '-d' that was dropped by CL:3684144.

BUG=None
TEST=compiles

Change-Id: I797f9995ad55d74e55d8c106ed42e61624c0f00f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3716014
Commit-Queue: David Stevens <stevensd@chromium.org>
Reviewed-by: Daniel Verkamp <dverkamp@chromium.org>
Tested-by: kokoro <noreply+kokoro@google.com>
diff --git a/src/crosvm/cmdline.rs b/src/crosvm/cmdline.rs
index dffe52f..6316534 100644
--- a/src/crosvm/cmdline.rs
+++ b/src/crosvm/cmdline.rs
@@ -504,6 +504,7 @@
     pub disable_virtio_intx: bool,
     #[argh(
         option,
+        short = 'd',
         long = "disk",
         arg_name = "PATH[,key=value[,key=value[,...]]",
         from_str_fn(numbered_disk_option)