Add support for VMs mounting shared host directories. Closes #57

When a local "folder" is shared, for example "/tmp" shared as
"my-tmp", then the shared folder can be mounted by the VM's OS by
issuing the following at the command line:

```shell
$ mount -t vboxfs my-tmp /mnt/tmp
```

This is done at the Hardware DSL level, by adding a new top level
entry :shared-folders that contains a vector of shares. Each share is
a vector of share name, host path, auto-mount?, writable?, the last
two being optional and defaulting to false and true (no auto-mount,
writable), e.g.

```clojure
{...
 :shared-folders [["my-tmp" "/tmp" nil true] ["my-var" "/var"]]
 ...}
```