t/qemu-runlinux: Use multidevs=remaps for 9P setup

Fixes the following warning that started to appear:

    kirr@deca:~/src/wendelin/wendelin.core/t$ ./qemu-runlinux -g  /home/kirr/src/linux/obj-qemu_debug/arch/x86/boot/bzImage /bin/bash
    qemu-system-x86_64: warning: 9p: Multiple devices detected in same VirtFS export, which might lead to file ID collisions and severe misbehaviours on guest! You should either use a separate export for each device shared from host or use virtfs option 'multidevs=remap'!

See https://wiki.qemu.org/Documentation/9psetup for documentation of
multidevs option.

t/qemu-runlinux

@@ -2,7 +2,7 @@
 # qemu-runlinux [options] <kernel> <program> ...
 # run kernel/program in QEMU with root fs taken from host
 #
-# Copyright (C) 2014-2019  Nexedi SA and Contributors.
+# Copyright (C) 2014-2021  Nexedi SA and Contributors.
 #                          Kirill Smelkov <kirr@nexedi.com>
 #
 # This program is free software: you can Use, Study, Modify and Redistribute
@@ -205,7 +205,7 @@ qemu-system-$arch   \
  \
     -m 512M `# default 128M is too limiting` \
  \
-    -fsdev  local,id=R,path=/,security_model=none,readonly  \
+    -fsdev  local,id=R,path=/,security_model=none,readonly,multidevs=remap  \
     -device virtio-9p-pci,fsdev=R,mount_tag=/dev/root   \
  \
     -kernel $kernel \