bug#47225: QEMU warning about performance

[Leo Famulari]

While using `guix system vm`, I noticed this warning. I think it's new,
maybe from the 5.2.0 update?

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qemu-system-x86_64: warning: 9p: degraded performance: a reasonable high msize 
should be chosen on client/guest side (chosen msize is <= 8192). See 
https://wiki.qemu.org/Documentation/9psetup#msize for details.
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That web page suggests that we increase the virtio packet size when
declaring the 9p file-systems, I think in %linux-vm-file-systems of (gnu
system vm).