block: Be more verbose in create fallback

For image creation code, we have central fallback code for protocols
that do not support creating new images (like NBD or iscsi).  So for
them, you can only specify existing paths/exports that are overwritten
to make clean new images.  In such a case, if the given path cannot be
opened (assuming a pre-existing image there), we print an error message
that tries to describe what is going on: That with this protocol, you
cannot create new images, but only overwrite existing ones; and the
given path could not be opened as a pre-existing image.

However, the current message is confusing, because it does not say that
the protocol in question does not support creating new images, but
instead that "image creation" is unsupported.  This can be interpreted
to mean that `qemu-img create` will not work in principle, which is not
true.  Be more verbose for clarity.

Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2217204


Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
Message-ID: <20230720140024.46836-1-hreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>