zpool status by default prints basenames of VDEVs, which means that GRUB
would have to go around guessing to see whether a VDEV exists. Instead,
it'd be more robust to simply tell zpool to give us full paths to VDEVs
via -P.
Signed-off-by: Arsen Arsenović <arsen@aarsen.me>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>