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]

Changed files
-3
nixos
modules
virtualisation
-3
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
···
--partition-guid=2:970C694F-AFD0-4B99-B750-CDB7A329AB6F \
--hybrid 2 \
--recompute-chs /dev/vda
${pkgs.dosfstools}/bin/mkfs.fat -F16 /dev/vda2
export MTOOLS_SKIP_CHECK=1
${pkgs.mtools}/bin/mlabel -i /dev/vda2 ::boot