Read/write access for passthrough (9p) filesystems with libvirt/qemu?
This seems to me like a permission issue on the host:
By default, the qemu/kvm process is started as a non-privileged user (libvirt-qemu in Debian Wheezy). So only files accessible (or writable) by that user are accessible (writable) by the VM guests.
You might try setting ownership of the directory to the user qemu is run as (see user= and group= in /etc/libvirt/qemu.conf)
Maybe this helps in your case?