wpa_supplicant.nix: Add option types
Add a priority level for overrides in VM tests

Now that overriding fileSystems in qemu-vm.nix works again, it's
important that the VM tests that add additional file systems use the
same override priority.  Instead of using the same magic constant
everywhere, they can now use mkVMOverride.

http://hydra.nixos.org/build/6695561
Drop the EFI installer tests from the release-critical builds for now

They fail randomly due to a hard-coded 30-second timeout in udev
waiting for /dev/sda1 to appear:

  systemd-udevd[1151]: worker [1168] /devices/pci0000:00/0000:00:04.0/host2/target2:0:0/2:0:0:0/block/sda/sda1 timeout; kill it
  systemd-udevd[1151]: seq 1059 '/devices/pci0000:00/0000:00:04.0/host2/target2:0:0/2:0:0:0/block/sda/sda1' killed

Hopefully we can use virtio in the future for the EFI tests.

http://hydra.nixos.org/build/6695897