Switch to 'mapped' access mode for hostfs

Passthrough mode only works as expected when running as root. On
Debian/Ubuntu systems qemu runs as user 'libvirt-qemu' and group 'kvm'
so all shared files must be chowned to grant access from guests.

Symlinks created on the host are still problematic because the Plan 9
filesystem has no direct notion of symbolic links, see [1].

[1] - http://ericvh.github.com/9p-rfc/rfc9p2000.u.html

testing/scripts/build-guestkernel

@@ -10,7 +10,6 @@ echo "Building guest kernel version $KERNELVERSION"
 
 check_commands bunzip2 bzcat make wget
 
-mkdir -p $BUILDDIR
 cd $BUILDDIR
 
 if [ ! -f "$KERNELTARBALL" ]