There is currently no way to set zone.cpu-shares for the global zone
(see 4970603). I have been using the attached transient SMF service 
to solve that until a final solution is available.

The service (svc:/site/global-zone) reads the amount of shares from 
the SMF repository and uses prctl(1M) to set zone.cpu-shares for the
global prior to any non-global zones being started by svc:/system/zones.
To persistently change the amount of CPU shares, use svccfg(1M) to set
the new value and refresh the service:

# svccfg -s svc:/site/global-zone setprop application/cpu-shares=50
# svcadm refresh svc:/site/global-zone

The new value will become effective immediately and will persist across reboots.