Addresses LP 1025353 for the v1 API.
Transition image status to active immediately on creation
(as opposed to leaving it queued forever) if the size is set
to zero from the get-go.
The v2 implementation is left unchanged for now, as the image
status does not appear to ever transition from queued to active
in that case.
This change allows an image to be created that simply acts as
a properties bucket, but requires no image data. For example,
an image created from a booted-from-volume instance where only
the kernel, ramdisk ID, and block device mappings are required.
Change-Id: I61e96f3fe5f5245fec791170b4a8b4c72135c3de
6873195cd1