daemon: Make StorageProvider accessible

And remove one obsolete prototype.

Reviewed-by: Stef Walter <stefw@redhat.com>
daemon: Use UDisks and storaged to check whether a block device is in use

Instead of trying to open it with O_EXCL, which doesn't work as
non-root.

The O_EXCL check is still effective, since wipefs will still do it
when it is called as root by UDisks.

Fixes #1205
Reviewed-by: Stef Walter <stefw@redhat.com>
test: Run storage tests as "admin"

Closes #1230
Reviewed-by: Stef Walter <stefw@redhat.com>