qemu-iotests: Treat custom TEST_DIR in 051

When custom TEST_DIR is specified the output includes it without leading
'/':

    $ TEST_DIR=/var/tmp ./check -file -qcow2 051
    ....
-drive0 (NODE_NAME): json:{"backing": {"driver": "qcow2", "file":
{"driver": "file", "filename": "TEST_DIR/t.qcow2"}}, "driver": "qcow2",
"file": {"driver": "file", "filename": SNAPSHOT_PATH}} (qcow2,
read-only)
+drive0 (NODE_NAME): json:{"backing": {"driver": "qcow2", "file":
{"driver": "file", "filename": "TEST_DIR/t.qcow2"}}, "driver": "qcow2",
"file": {"driver": "file", "filename": "TEST_DIR/vl.ziHfeP"}} (qcow2,
read-only)

Let's remove it from the sed regexp.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

tests/qemu-iotests/051

@@ -360,7 +360,7 @@ TMPDIR=/nonexistent run_qemu -drive driver=null-co,snapshot=on
 echo "info block" |
     run_qemu -drive file="$TEST_IMG",snapshot=on,read-only=on,if=none,id=$device_id |
     _filter_qemu_io |
-    sed -e 's#"/[^"]*/vl\.[A-Za-z0-9]\{6\}"#SNAPSHOT_PATH#g'
+    sed -e 's#"[^"]*/vl\.[A-Za-z0-9]\{6\}"#SNAPSHOT_PATH#g'
 
 
 # success, all done