Updates for large sets of content in /var and virtiofsd option to libvirt run

[nickryand]

There are two changes here:

1. Swap /var over to an overlay. I ran into issues when I was trying to build a container that pre-installed a large set of content on /var. The copy that was happening from a read-only mount to the tmpfs during the vm hybrid build phase was having issues and causing that step to hang and SSH would never start. This change removes the tmpfs and sets up an overlayfs mount that disables the inode indexing and metacopy.

This allows for /var/ to be customized via standard Containerfile with large artifacts. The bcvk build process does not run into any issues in my testing. It also requires less memory since there is not a tmpfs for /var that has data copied into it.

2. Adding the --virtiofsd option to libvirt run. I forgot to upstream this after Add --virtiofsd CLI option and VIRTIOFSD_BIN env var support #266.