- kvm_server pattern:
- require the basic libvirt modules and packages for a minimal
(but properly working) KVM host
- remove the direct dependency for qemu, as that now comes from
libvirt-daemon-driver-qemu
This makes the kvm_server pattern a lot more useful, in contexts
where recommended packages are not installed by default. On the
other hand, everyone using it with recommended packages installation
enabled, will see no difference at all.