/usr/lib/xen-common/bin/xen-dir is in xen-utils-common 4.1.4-3+deb7u9.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10  | #!/bin/sh -e
VERSION=$(. /usr/lib/xen-common/bin/xen-version); RET=$?; [ $RET -eq 0 ] || exit $RET
if [ -d "/usr/lib/xen-$VERSION" ]; then
    echo "/usr/lib/xen-$VERSION"
else
    echo "ERROR:  Can't find version $VERSION of xen utils, bailing out!" >&2
    exit 127
fi
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