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".