feat: Qemu exporters minor improvements

- Improve documentation about vm configuration.
- Change error management to warn user if he forgot about the --vm
  switch.

.gitattributes

@@ -0,0 +1 @@
+*.png binary

docs/exporters/qemu.md

@@ -17,24 +17,37 @@ First create a tmpfs mount point to isolate metrics for that virtual machine:
 		mount -t tmpfs tmpfs_DOMAIN_NAME /var/lib/libvirt/scaphandre/DOMAIN_NAME -o size=10m
 
 3. Ensure you expose the content of this folder to the virtual machine by having this configuration in the xml configuration of the domain:
-
+```
 		<filesystem type='mount' accessmode='passthrough'>
 	      <driver type='virtiofs'/>
 	      <source dir='/var/lib/libvirt/scaphandre/DOMAIN_NAME'/>
 	      <target dir='scaphandre'/>
 		  <readonly />
 	    </filesystem>
+```
+You can edit the vm properties using `sudo virsh edit <DOMAIN_NAME>` using your usual editor. But it is more convenient to use virtual-manager, as explained in the following screenshots.
+
+*It also helps to define the correct syntax which probably depends from the qemu version. You can check that the above configuration is slightly different form the one below*.
+
+a. Right click in the hardware menu:
+![virtualmgr00](images/virtualmgr00.png)
+
+b. Enter the following parameters:
+![virtualmgr01](images/virtualmgr01.png)
+
+c. XML generated as a result:
+![virtualmgr02](images/virtualmgr02.png)
 
-3. Ensure the VM has been started once the configuration is applied, then mount the filesystem on the VM/guest:
+4. Ensure the VM has been started once the configuration is applied, then mount the filesystem on the VM/guest:
 
 		mount -t 9p -o trans=virtio scaphandre /var/scaphandre
 
-4. Still in the guest, run scaphandre in VM mode with the default sensor:
+5. Still in the guest, run scaphandre in VM mode with the default sensor:
 
 		scaphandre --vm prometheus
 
-5. Collect your virtual machine specific power usage metrics. (requesting http://VM_IP:8080/metrics in this example, using the prometheus exporter)
+6. Collect your virtual machine specific power usage metrics. (requesting http://VM_IP:8080/metrics in this example, using the prometheus exporter)
 
 As always exporter's options can be displayed with `-h`:
 
-	scaphandre qemu -h
+	scaphandre qemu -h

src/sensors/mod.rs

@@ -235,7 +235,7 @@ impl Topology {
                 .sockets
                 .iter_mut()
                 .find(|x| &x.id == socket_id)
-                .unwrap();
+                .expect("Trick: if you are running on a vm, do not forget to use --vm parameter invoking scaphandre at the command line");
             if socket_id == &socket.id {
                 socket.add_cpu_core(c);
             }