drop libvirt package dependency

There is no need to add a libvirt dependency in our module, since it's
already handled by puppet-nova.

Plus, it causes issues on Red Hat where libvirt package is not
'libvirt-bin'.

Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
Merge pull request #430 from enovance/libvirt-dep

drop libvirt package dependency