libzfsbootenv: lzbe_nvlist_set needs to store bootenv version VB_NVLIST

A small bug did slip into initial libzfsbootenv; while storing nvlist
in nvlist, we should make sure the bootenv is using VB_NVLIST format.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Toomas Soome <tsoome@me.com>
Closes #10937