dev-virtio: use diod basename as the default 9p path

dev-virtio: use diod basename as the default 9p path

This allows diod to be present anywhere in the PATH by default,
which works because we are already using execlp.

Change-Id: I9d0b6c9a75f32cf0cb5d8f52bb00c465e4d43e1b
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22827
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Jason Lowe-Power <jason@lowepower.com>