vivian: add virtiofs pieces for mounting a local share

With a linux image built with the support provided at mupuf/boot2container!43 (merged), we could now do:

$ make VIVIAN_OPTS="--local-share=<path_to_local_folder>" vivian

And then, inside vivian, mount the shared folder like:

[root@vivian ~]# mkdir -p /mnt/virtiofs && mount -t virtiofs myfs /mnt/virtiofs