newvm: Use fedora-rawhide OS variant

Apparently, it's not guaranteed that _libosinfo_ always supports even
the version of Fedora it's installed on: there's no _fedora42_ in
_libosinfo-1.12.0-2.fc42_ 🤦🏻‍♂️.

Fortunately, it makes almost no difference what OS variant is selected
at install time, and we probably want the latest features anyway.  Thus,
we can just use _fedora-rawhide_ instead of any particular version and
not worry about it.

newvm.sh

@@ -182,7 +182,7 @@ set -- \
     --extra-args "${extra_args}" \
     --disk pool=default,size=${disk_size},cache=none \
     --network ${network} \
-    --os-variant fedora$(rpm -E %fedora) \
+    --os-variant fedora-rawhide \
     --console pty,target.type=serial,log.file=/var/log/libvirt/console/${name}.log \
     --sound none \
     --redirdev none \