Add pm-utils to default libvirt packages on Ubuntu

pm-utils is *suggested* by aptitude when installing libvirt-bin (since 0.9.8-2ubuntu17.4), but the apt options in our cookbooks won't automatically install suggested packages. If you guys think the logic for this belongs somewhere else in the cookbook I'll be happy to make the appropriate changes.

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Without pm-utils installed, the libvirt daemon will spam its error
log with "Cannot find 'pm-is-supported'" messages every second.

Bug: https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/994476