iotests: Forbid 020 for non-file protocols

This test does funny things like TEST_IMG="TEST_IMG.base" _make_test_img
that usually only work with the file protocol.  More specifically, they
do not work with the most interesting non-file protocols, so we might as
well skip this for anything but file.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Message-id: 20171123020832.8165-8-mreitz@redhat.com
Reviewed-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NMax Reitz <mreitz@redhat.com>

tests/qemu-iotests/020

@@ -42,18 +42,12 @@ trap "_cleanup; exit \$status" 0 1 2 3 15
 
 # Any format supporting backing files
 _supported_fmt qcow qcow2 vmdk qed
-_supported_proto generic
-_unsupported_proto vxhs
+_supported_proto file
 _supported_os Linux
 _unsupported_imgopts "subformat=monolithicFlat" \
                      "subformat=twoGbMaxExtentFlat" \
                      "subformat=twoGbMaxExtentSparse"
 
-# NFS does not support bdrv_reopen_prepare thus qemu-img commit fails.
-if [ "$IMGPROTO" = "nfs" ]; then
-    _notrun "image protocol $IMGPROTO does not support bdrv_commit"
-fi
-
 TEST_OFFSETS="0 4294967296"
 
 TEST_IMG_SAVE="$TEST_IMG"