If the system didn't build the dev link, you'll need to run
devfsadm -v to build. I also like to toss in
-C to also clean up the dev tree. Aslo note that the dev tree on x86 is slightly different than SPARC for storage.
As Jeff Schaller mentions in a comment, mirroring to another drive would not give you any additional space. That would/may give you some redundancy (assuming the 2nd drive served via VirtualBox is another device), so it might only help with ZFS being able to possibly repair data corruption.
If your goal is for more storage, you may just want to add a drive via virtualbox and create another zpool on it and create datasets on it as needed.
I haven't tried it, but I believe you can "grow" the storage on the virtualbox side. If so, and you have the "autoexpand" option set on the pool, the pool will try to grow if there is more storage given on that slice.
zpool get autoexpand rpool
NAME PROPERTY VALUE SOURCE
rpool autoexpand off default
Another option is to enable compression on some of the datasets to help with data growth. Just note, that currently on the boot dataset for the OS, you can't have a compression higher than "on," but I believe there is a RFE for this to be changed.