nixos/qemu-vm: Disable cache for $NIX_DISK_IMAGE

As @domenkozar noted in #10828, cache=writeback seems to do more harm
than good:

https://github.com/NixOS/nixpkgs/issues/10828#issuecomment-164426821

He has tested it using the openstack NixOS tests and found that
cache=none significantly improves startup performance.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

nixos/modules/virtualisation/qemu-vm.nix

@@ -76,14 +76,14 @@ let
           -virtfs local,path=$TMPDIR/xchg,security_model=none,mount_tag=xchg \
           -virtfs local,path=''${SHARED_DIR:-$TMPDIR/xchg},security_model=none,mount_tag=shared \
           ${if cfg.useBootLoader then ''
-            -drive index=0,id=drive1,file=$NIX_DISK_IMAGE,if=${cfg.qemu.diskInterface},cache=writeback,werror=report \
+            -drive index=0,id=drive1,file=$NIX_DISK_IMAGE,if=${cfg.qemu.diskInterface},cache=none,werror=report \
             -drive index=1,id=drive2,file=$TMPDIR/disk.img,media=disk \
             ${if cfg.useEFIBoot then ''
               -pflash $TMPDIR/bios.bin \
             '' else ''
             ''}
           '' else ''
-            -drive index=0,id=drive1,file=$NIX_DISK_IMAGE,if=${cfg.qemu.diskInterface},cache=writeback,werror=report \
+            -drive index=0,id=drive1,file=$NIX_DISK_IMAGE,if=${cfg.qemu.diskInterface},cache=none,werror=report \
             -kernel ${config.system.build.toplevel}/kernel \
             -initrd ${config.system.build.toplevel}/initrd \
             -append "$(cat ${config.system.build.toplevel}/kernel-params) init=${config.system.build.toplevel}/init regInfo=${regInfo} ${kernelConsole} $QEMU_KERNEL_PARAMS" \