Move ImageTooLarge check to Compute API

The existing ImageTooLarge check is in the compute manager, meaning that a
validation error puts the instance into an ERROR state without the ability to
communicate the fault back to the user.

This patch moves the check to the API where we can return the appropriate
error to the user.

Change-Id: I2183449351e3b70f98d0552ea247184a3641d85a
Merge "Move ImageTooLarge check to Compute API"