qemu-iotests: add support for the vdi image format

Signed-off-by: NChristoph Hellwig <hch@lst.de>

tests/qemu-iotests/common

@@ -120,6 +120,7 @@ check options
     -cow                test cow
     -qcow               test qcow
     -qcow2              test qcow2
+    -vdi                test vdi
     -vpc                test vpc
     -vmdk               test vmdk
     -xdiff		graphical mode diff
@@ -158,6 +159,11 @@ testlist options
 	    xpand=false
 	    ;;
 
+	-vdi)
+	    IMGFMT=vdi
+	    xpand=false
+	    ;;
+
 	-vmdk)
 	    IMGFMT=vmdk
 	    xpand=false