touch reboot-required in postinst if running xen

When updating the xen hypervisor, touch the /run/reboot-required file to indicate a reboot is required. Also add information about the package requesting the reboot to the /run/reboot-required.pkgs file.

It is done according to https://www.debian.org/doc/debian-policy/ch-opersys.html#signaling-that-a-reboot-is-required and also fixes https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=862408