/lib/udev/rules.d/xend.rules is in xen-utils-common 4.4.0-0ubuntu5.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 | #SUBSYSTEM=="pci", RUN+="socket:/org/xen/xend/udev_event"
#SUBSYSTEM=="scsi", RUN+="socket:/org/xen/xend/udev_event"
#SUBSYSTEM=="usb", RUN+="socket:/org/xen/xend/udev_event"
#SUBSYSTEM=="net", KERNEL!="vif[0-9]*.[0-9]*|tap[0-9]*.[0-9]*", RUN+="socket:/org/xen/xend/udev_event"
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