qemu-iotests: Test 137 only supports 'file' protocol

Since test 137 make uses of qcow2.py, only local files are supported.
Signed-off-by: NJeff Cody <jcody@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

tests/qemu-iotests/137

@@ -39,7 +39,7 @@ trap "_cleanup; exit \$status" 0 1 2 3 15
 . ./common.qemu
 
 _supported_fmt qcow2
-_supported_proto generic
+_supported_proto file
 _supported_os Linux