Increase buffer size for nvlist for large configurations

Increase buffer size for nvlist for large configurations
It's still not clear to me why the default value here is large
enough Solaris.  I hit this limit again when setting up 120 SATA
drives configured as 15 raidz2 groups each containing 8 drives.
We expect to go bigger so we may just want to spend a little
time and figure out how to make this all dynamic.