Pull out instance object handling for use by create also

The instance_create() function needs the same type handling
logic to tolerate object-initiated creates as does instance_update().
This pulls that logic out to a common function and makes both of
them use it.

Related to blueprint compute-api-objects

Change-Id: Ie8612ffb225b95bf7eab3c462441665095ed19bc
Merge "Pull out instance object handling for use by create also"