Expose vhost vdpa device to container

[alxiord]

To unblock rust-vmm/vhost#33, several conditions must be met:

1. CI kernel version should be >= 5.7 (currently 5.4)
2. vhost vdpa kernel modules should be loaded (see add vhost-vdpa support (in-kernel) vhost#33 (comment))
3. the /dev/vhost-vdpa-0 device should be exposed to the container (see add vhost-vdpa support (in-kernel) vhost#33 (comment))

1 and 2 can be fixed by updating the AMIs for the CI runners. This issue tracks step 3, for which we need to update the Buildkite pipeline so that the steps that involve running the unit tests can access the device. The device needs to be passed in the Buildkite yaml as devices: [ "/dev/vhost-vdpa-0" ] (see documentation).