feat: add redis qemu exporter and sensor

[AldericoGallo]

Add a new QEMU exporter that allows publishing the power consumption of VMs to a Redis server, the VMs can get the metric using the Redis sensor.

This allows to use Scaphandre on OpenStack and other QEMU based hypervisor without changing the QEMU or libvirt config.

Other changes included:

• Fixed Proxmox support, should be the same as the changes made in Add support for native KVM driver VMs in the Qemu exporter #389
• Allow setting the step between measurement on the qemu exporter
• Allow creating a topology that uses the sum of cpu usage for calculating the power consumption, example:
  A VM with 4 core has a process that uses 1 core at 100%, normally scaphandre consider a 25% CPU usage for that process to calculate the power consumption but considering how the VM total power consumption is calculated it make more sense to consider that process as using 100% of the CPU (if it's the only one running), this may need more testing to understand if it's a better estimation.
  In this PR this is only used for the new redis sensor.