let Libvirt automatically select best CPU model

The CPU model and vendor of a VM was hard-coded in cloubed. This prevented
cloubed from booting VM on hosts with different CPU. Now, cloubed specifies
in the XML representation of a Domain (aka. VM) that Libvirt should boot the VM
with a CPU identical to the host and enable all its features (eg. vmx).

Fixes #2.
let Libvirt find path to KVM executable

The path to KVM executable was hard-coded in cloubed. Unfortunately, this path
changes from one GNU/Linux distribution to another. Now, Libvirt is responsible
to find KVM executable itself through its capabilities feature.

Fixes #1.