Install acpid in the CentOS guest image

Remove the note about 'virsh reboot' not working. This is not a bug,
this is a feature (kind of).

Document the installation of acpid on the guest system.
Made edits for consistency.

Change-Id: I5462a128a0e8876fd2ec9e9fee66f1d2e662ccf4
Closes-Bug: #1337242
Merge "Install acpid in the CentOS guest image"