Update hftest.sh to pass correct argument to hftest.py for FVP.

This was changed in hftest.py in
https://hafnium-review.googlesource.com/c/hafnium/+/8560 but not
updated here.

Change-Id: I795bac11aed125fc47606e26d8cad2d5803cfe9c
diff --git a/kokoro/test.sh b/kokoro/test.sh
index 0dcd187..c35f5c2 100755
--- a/kokoro/test.sh
+++ b/kokoro/test.sh
@@ -55,7 +55,7 @@
 HFTEST=(${TIMEOUT[@]} 300s ./test/hftest/hftest.py)
 if [ $USE_FVP == true ]
 then
-  HFTEST+=(--fvp)
+  HFTEST+=(--driver=fvp)
   HFTEST+=(--out "$OUT/aem_v8a_fvp_clang")
   HFTEST+=(--out_initrd "$OUT/aem_v8a_fvp_vm_clang")
 else