Adjust 'ceph' authentication secret usage for rbd pool.

Adjust 'ceph' authentication secret usage for rbd pool.

Update virStorageBackendRBDOpenRADOSConn() to use the internal API to the
secret driver in order to get the secret value instead of the external
virSecretGetValue() path. Without the flag VIR_SECRET_GET_VALUE_INTERNAL_CALL
there is no way to get the value of private secret.

This also requires ensuring there is a connection which wasn't true for
for the refreshPool() path calls from storageDriverAutostart() prior to
adding support for the connection to a qemu driver. It seems calls to
virSecretLookupByUUIDString() and virSecretLookupByUsage() from the
refreshPool() path would have failed with no way to find the secret - that is
theoretically speaking since the 'conn' was NULL the failure would have been
"failed to find the secret".