nixos/qemu-vm: Don't explicitly mknod /dev/vda*

And don't need to source the uevent files anymore either since $MAJOR
or $MINOR aren't used elsewhere.

[dezgeg: The reason these are no longer needed is that 0d27df280f7ed5
switched /tmp to a devtmpfs which automatically creates such device
nodes]

nixos/modules/virtualisation/qemu-vm.nix

···
             --partition-guid=2:970C694F-AFD0-4B99-B750-CDB7A329AB6F \
             --hybrid 2 \
             --recompute-chs /dev/vda
-          . /sys/class/block/vda2/uevent
-          mknod /dev/vda2 b $MAJOR $MINOR
-          . /sys/class/block/vda/uevent
           ${pkgs.dosfstools}/bin/mkfs.fat -F16 /dev/vda2
           export MTOOLS_SKIP_CHECK=1
           ${pkgs.mtools}/bin/mlabel -i /dev/vda2 ::boot