Tweaks to definition.rb to use correct shutdown command.

postinstall.sh -> postinstall.csh since it's really going to be run by
csh. Fixes in the file to ensure "fetch" command is used correctly. Also
added virtio kmod build so that virtualbox virtio network interface can
be used.
Merge pull request #803 from lukebakken/freebsd-fixes

Fixes for FreeBSD 9.1 template files.