executor: add support for network block devices
This MR enables jobs to create empty or url-backed network block devices. This is particularly useful for diskless DUTs as it enables them to store files without using their precious RAM, but it can also be useful when trying to boot ready-made distro ISOs.
This MR is limiting itself to non-persistent storage, which means jobs cannot reuse an NBD disk created by another job.
References: #238 Cc: @austriancoder