qemu-iotests: only run 016 for file and sheepdog protocols

016 writes past EOF which isn't support by most protocols, so limit
it to file and sheepdog, which explicitly support it.

Pointed out by Josh Durgin <josh.durgin@dreamhost.com>.
Signed-off-by: NChristoph Hellwig <hch@lst.de>

tests/qemu-iotests/016

@@ -38,9 +38,8 @@ trap "_cleanup; exit \$status" 0 1 2 3 15
 . ./common.rc
 . ./common.filter
 
-# currently raw is the only protocol we support
 _supported_fmt raw
-_supported_proto generic
+_supported_proto file sheepdog
 _supported_os Linux