block: Allow drivers to control protocol prefix at creation

block: Allow drivers to control protocol prefix at creation

This patch is pure refactoring: instead of hard-coding permission to
use a protocol prefix when creating an image, the drivers can now pass
in a parameter, comparable to what they could already do for opening a
pre-existing image.  This patch is purely mechanical (all drivers pass
in true for now), but it will enable the next patch to cater to
drivers that want to differ in behavior for the primary image vs. any
secondary images that are opened at the same time as creating the
primary image.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-ID: <20250915213919.3121401-5-eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>