CI: wait for the network interface in PCI passthrough tests

The network driver initializes asynchronously, and it may not be ready
yet by the time the startup script is called. This is especially the
case for USB network adapter (where the PCI device is the USB
controller) in the upcoming runner.

Don't bother about separate timeout - test timeout will cover this part
too.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

automation/scripts/qubes-x86-64.sh

@@ -104,6 +104,7 @@ on_reboot = "destroy"
         domU_check="
 set -x -e
 interface=eth0
+while ! [ -e \"/sys/class/net/\$interface\" ]; do sleep 1; done
 ip link set \"\$interface\" up
 timeout 30s udhcpc -i \"\$interface\"
 pingip=\$(ip -o -4 r show default|cut -f 3 -d ' ')