This cleans up the handling of image size in img_create() by parsing
the value early, and then only setting it once if a value has been
added as the last argument to the command line.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>