dev, virtio: Improvements to diod process handling

dev, virtio: Improvements to diod process handling

* When dispatching multiple gem5 simulations at once, they race
for the socket id, resulting in a panic when calling 'bind'. To
avoid this problem, the socket id is now created before the diod
process is created.  In case of a race, a panic is called in the
gem5 process, whereas before the panic was called in the diod
process where it didn't have any effect.

* In some cases killing the diod process in terminateDiod() using
only SIGTERM failed, so a call using SIGKILL is added.

Change-Id: Ie10741e10af52c8d255210cd4bfe0e5d761485d3
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Sascha Bischoff <sascha.bischoff@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/2821
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>