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>

src/dev/virtio/VirtIO9P.py

@@ -59,7 +59,7 @@ class VirtIO9PDiod(VirtIO9PProxy):
     type = 'VirtIO9PDiod'
     cxx_header = 'dev/virtio/fs9p.hh'
 
-    diod = Param.String("/usr/sbin/diod", "Path to diod")
+    diod = Param.String("diod", "Path to diod, optionally in PATH")
     root = Param.String("/tmp", "Path to export through diod")
     socketPath = Param.String("Unused socket to diod")