Improve instructions for setting up VM to debug MM setup manually

* Use a MAC address in the example that can actually be used but that also
  does not conflict with the range we assign in os-autoinst for SUT VMs
* Ensure that the device is up upfront and combine that with setting the
  MTU
* Make it clear that the tap device must not be used by a SUT-VM at the
  same time
* See https://progress.opensuse.org/issues/152389

docs/Networking.asciidoc

@@ -214,17 +214,21 @@ for MM jobs:
 ```
 wget http://download.opensuse.org/tumbleweed/appliances/openSUSE-Tumbleweed-Minimal-VM.x86_64-Cloud.qcow2
 qemu-system-x86_64 -m 2048 -enable-kvm -vnc :42 -snapshot \
-  -netdev tap,id=qanet0,ifname=tap39,script=no,downscript=no -device virtio-net,netdev=qanet0,mac=00:00:00:00:00:01 \
+  -netdev tap,id=qanet0,ifname=tap40,script=no,downscript=no -device virtio-net,netdev=qanet0,mac=52:54:00:13:0b:4a \
   openSUSE-Tumbleweed-Minimal-VM.x86_64-Cloud.qcow2
 ```
 
-The image used here is of course just an example. Within the VM configure the network
-*like* this (you may need to adjust concrete IP addresses, subnets and interface names):
+The image used here is of course just an example. You need to make sure to assign a unique
+MAC address (e.g. by adjusting the last two figures in the example; this will not conflict with
+MAC addresses used by os-autoinst) and use a tap device not used at the same time by a SUT-VM.
+
+Within the VM configure the network *like* this (you may need to adjust concrete IP addresses,
+subnets and interface names):
 
 ```
+ip link set dev eth0 up mtu 1458
 ip a add dev eth0 10.0.2.15/24
 ip r add default via 10.0.2.2
-ip link set dev eth0 mtu 1458
 echo 'nameserver 8.8.8.8' > /etc/resolv.conf
 ```