config: support overriding the guest directory with HOST_DIR set

[mathstuf]

Fixes: #145

[mathstuf]

This fails with mount_virtiofs: failed to mount /OUR/CUSTOM/PATH: Operation not permitted. Needs more work.

[mathstuf]

So investigating, it looks like things are hooked up with:

• --dir offers the host-side directory for mounting and tags it with tart.virtiofs.*
• when setting up the VM, this triggers commands to do mkdir -p $path on the VM-side path and to call mount_virtiofs tart.virtiofs.* $path

Need to do some manual testing to learn why our custom path is denied while /var/tmp/* is accepted.

[Brett-Best]

@mathstuf just FYI, I believe that on macOS 27b3 this is hardened further and /var/tmp/* is no longer accepted. I need to look into that further.

[mathstuf]

Oh fun. Yeah, still on 26 here.

[mathstuf]

Ah! The Tart VM bits missed the "create the directory". Let me respin images…

[mathstuf]

Images with the requested directory and the correct permissions work with this change. Tested in our CI setup.