d/t/qemu-net-iscsi: determinate boot by LABEL

Benjamin Drungrequested to merge
bdrung/initramfs-tools:deterministic-boot into debian/latest

running qemu-net-iscsi can end up with an incorrect boot order on s390x. This happens during the first qemu boot when two drives are provided to QEMU. By labeling the filesystem and setting the kernel cmdline root= to LABEL=rootdisk, boot order is explicitly stated.

rather than overloading ROOTDISK_LINUX_NAME, have run_qemu_in_background take another parameter setting the kernel cmdline rootdisk. This makes it more flexible. Patch qemu-net-* tests to call it appropriately, and patch iscsi tests to use SERVER_ROOTDISK_LINUX_NAME to make a consistent label.

LP: #2134531

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