dev: Replace Callback in the virtio device with a lambda.

dev: Replace Callback in the virtio device with a lambda.

Issue-on: https://gem5.atlassian.net/browse/GEM5-698
Change-Id: Ia628ceb0080b11b81c7eee82e7c8c0049b2cd62f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32649
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>