VM tests: Run hwclock in the initrd

Needed for the installer tests, since otherwise mounting a filesystem
may fail as it has a last-mounted date in the future.

http://hydra.nixos.org/build/9846712

nixos/modules/virtualisation/qemu-vm.nix

@@ -291,6 +291,11 @@ in
         if test -z "$FSTYPE"; then
             mke2fs -t ext4 /dev/vda
         fi
+
+        # Set the system time from the hardware clock to work around a
+        # bug in qemu-kvm > 1.5.2 (where the VM clock is initialised
+        # to the *boot time* of the host).
+        hwclock -s
       '';
 
     boot.initrd.postMountCommands =