Install upower and consolekit if not already installed:
pacman -S upower consolekit
Run pkaction and you should see these two among the... err... things listed:
org.freedesktop.consolekit.system.stop
org.freedesktop.upower.suspend
Create/edit custom-actions.pkla:
nano /var/lib/polkit-1/localauthority/50-local.d/custom-actions.pkla
"Type" in:
[Actions for xbmc user]where xbmc is the user you have running xbmc.
Identity=unix-user:xbmc
Action=org.freedesktop.upower.*;org.freedesktop.consolekit.system.*
ResultAny=yes
ResultInactive=yes
ResultActive=yes
That's it.