Add a privileged field to storageDriverState

Use the privileged value in order to generate a connection which could
be passed to the various storage backend drivers.

In particular, the iSCSI driver will need a connect in order to perform
pool authentication using the 'chap' secrets and the RBD driver utilizes
the connection during pool refresh for pools using 'ceph' secrets.

For now that connection will be to be to qemu driver until a mechanism
is devised to get a connection to just the secret driver without qemu.