Remove unnecessary db call in scheduler driver live-migration code

When the scheduler selects a destination host for live-migration, a db
call is made to get instance_type info for the instance. To avoid the
db call, this should instead call instance_types.extract_instance_type
on the instance.

Resolves bug 1167811.

Change-Id: I2025fdf1d34e70158360ff71e8545c8e3bc6bc80