tests: virtme: Fix running on fedora 40

Virtiofs is used by default to mount the rootfs in virtme-ng.

In Fedora 40 virtiofs support seems broken with virtme-ng,
so force using 9p instead.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7455>

tests/virtme/virtme-run.sh

@@ -59,7 +59,7 @@ virtme-run \
   --script-sh "sh -c \"$SCRIPT\"" \
   -a visl.stable_output=true \
   -a visl.codec_variability=true \
-  --show-boot-console --show-command \
+  --show-boot-console --show-command --force-9p \
   --qemu-opts -cpu host,pdcm=off -smp 8
 VM_RESULT=$?
 if [ $VM_RESULT != 0 ]; then