This commit adds an Error ** argument to bdrv_img_create() and set it
appropriately on error.

Callers of bdrv_img_create() pass NULL for the new argument and still
rely on bdrv_img_create()'s return value. Next commits will change
callers to use the Error object instead.
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>