qemu: warn that network won't work with ui=none

The other day I spent way too long trying to find a regression that
caused network inside qemu not to work anymore, before I realized it was
caused by selecting UI=none instead of anything else.

Print a warning and link to a wiki page describing this in more detail.

Reviewed-by: Clayton Craft <clayton@craftyguy.net>
Link: https://lists.sr.ht/~postmarketos/pmbootstrap-devel/%3C20230226184546.6869-1-ollieparanoid@postmarketos.org%3E

pmb/qemu/run.py

@@ -340,7 +340,7 @@ def run(args):
         logging.info("NOTE: Run 'pmbootstrap qemu --image-size 2G' to set"
                      " the rootfs size when you run out of space!")
 
-    # SSH/serial hints
+    # SSH/serial/network hints
     logging.info("Connect to the VM:")
     logging.info("* (ssh) ssh -p {port} {user}@localhost".format(**vars(args)))
     logging.info("* (serial) in this console (stdout/stdin)")
@@ -351,6 +351,10 @@ def run(args):
         logging.info("NOTE: To quit QEMU with this option you can use "
                      "Ctrl-A, X.")
 
+    if args.ui == "none":
+        logging.warning("WARNING: With UI=none network doesn't work"
+                        " automatically: https://postmarketos.org/qemu-network")
+
     # Run QEMU and kill it together with pmbootstrap
     process = None
     try: