Add pm-utils to default libvirt packages on Ubuntu

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
Merge pull request #231 from rsoprivatecloud/libvirt-pm-utils

Add pm-utils to default libvirt packages on Ubuntu