After a reboot the ZFS pool is not mounted but it can be mounted manually using the "zpool import" command.
If disks being presented are shown as "pci" then they will not be imported during the reboot, we will not import "pci" disks by default.
Verify your VirtualBox configuration and see what type of controller it is configured for.
It should be presenting disks as "Serial Attached SCSI (SAS)" and emulating an LSI Logic SAS controller so that we detect them correctly and are able to manage them.
This link contains additional detail on VertualBox configurations: