Fix the generalized copy_image_to_volume operation.

The Generis iSCSI copy volume<->image patch
(change: Iff097629bcce9154829a7eb5aee0ea6302338b26) did
not account for the fact that the existing volume reference
passed in to the copy method would not have the updated
provider_location and iSCSI info available.

To address this, we simply just get a new ref from the DB
after creation if a copy image is requested.

Also, the list of acceptable formats was not fully inclusive,
not only that, but there's no reason to not let the qemu-convert
handle any errors here.

Change-Id: I7b1910080d285562c7f2d8afa7f65768b1089b73