configs: Use absolute path for VirtIO9PDiod default root

configs: Use absolute path for VirtIO9PDiod default root

VirtIO9PDiod model requires an absolute path as its parameter. So we
should change the default root path to absolute path as well.

Change-Id: I68a2ae1115e84ed61055298b06b2d0b4bd6410b3
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/36381
Reviewed-by: Earl Ou <shunhsingou@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>