virtualbox: Fix extension pack installation.

With hardening, we need to go a bit further rather than just allowing
/nix/store being world-writable. We now use fakeroot to make sure the
VBoxExtPackHelperApp won't moan that the files are not owned by root.

They are, but only outside of the chrooted build process.

Another issue with using fakeroot is that it doesn't seem to cope well
with arguments that contain spaces. That's why I've piped the call into
${stdenv.shell}.

Now, the really gory and confusing part is the introduction of
VBOX_PATH_APP_PRIVATE_ARCH_TOP and the change of VBOX_PATH_APP_PRIVATE.

The VBOX_PATH_APP_PRIVATE_ARCH is *only* for modules and is checked by
the hardened implementation against whether things like VMMR0.r0 or
VBoxVMM.so reside in that directory. As a side note: I admit that the
whole libexec directory is quite polluted with stuff that shouldn't be
there, but for now we've broken enough things and will tear apart the
whole structure at some day in the future[TM].

For the confusing part we have VBOX_PATH_APP_PRIVATE_ARCH_TOP, which
_should_ be the same as VBOX_PATH_APP_PRIVATE_ARCH but unfortunately,
the hardened implementation is checking against this directory (in
IsValidBaseDir) for the extension pack(why!?).

Of course, we could put even that into the libexec directory, somewhat
similar as the official package, but after all, let's at least *try* to
separate things.

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