Install acpid in the CentOS guest image

Remove the note about 'virsh reboot' not working.
Document the installation of acpid on the guest system.

This restores changes accidentally dropped from review 106560.

Change-Id: I447506ef65169c8f3cd03f376e96fbc005aafe65
Closes-Bug: #1337242
Merge "Install acpid in the CentOS guest image"