iotests: Drop IMGOPTS use in 267

Overwriting IMGOPTS means ignoring all user-supplied options, which is
not what we want.  Replace the current IMGOPTS use by a new BACKING_FILE
variable.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-id: 20191107163708.833192-14-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>

tests/qemu-iotests/267

@@ -69,7 +69,11 @@ size=128M
 
 run_test()
 {
-    _make_test_img $size
+    if [ -n "$BACKING_FILE" ]; then
+        _make_test_img -b "$BACKING_FILE" $size
+    else
+        _make_test_img $size
+    fi
     printf "savevm snap0\ninfo snapshots\nloadvm snap0\n" | run_qemu "$@" | _filter_date
 }
 
@@ -120,12 +124,12 @@ echo
 
 TEST_IMG="$TEST_IMG.base" _make_test_img $size
 
-IMGOPTS="backing_file=$TEST_IMG.base" \
+BACKING_FILE="$TEST_IMG.base" \
 run_test -blockdev driver=file,filename="$TEST_IMG.base",node-name=backing-file \
          -blockdev driver=file,filename="$TEST_IMG",node-name=file \
          -blockdev driver=$IMGFMT,file=file,backing=backing-file,node-name=fmt
 
-IMGOPTS="backing_file=$TEST_IMG.base" \
+BACKING_FILE="$TEST_IMG.base" \
 run_test -blockdev driver=file,filename="$TEST_IMG.base",node-name=backing-file \
          -blockdev driver=$IMGFMT,file=backing-file,node-name=backing-fmt \
          -blockdev driver=file,filename="$TEST_IMG",node-name=file \
@@ -142,7 +146,7 @@ echo
 echo "=== -blockdev with NBD server on the backing file ==="
 echo
 
-IMGOPTS="backing_file=$TEST_IMG.base" _make_test_img $size
+_make_test_img -b "$TEST_IMG.base" $size
 cat <<EOF |
 nbd_server_start unix:$SOCK_DIR/nbd
 nbd_server_add -w backing-fmt