conf: Adjust invalid secrettype setting during parse

conf: Adjust invalid secrettype setting during parse

Commit id '1feaccf0' attempted to handle an empty secrettype value; however,
it made a mistake by processing the secretType as if it was the original
secrettype string.  The 'secretType' is actually whether 'usage' or 'uuid'
was used.

Thus adjust part of the change to make the same check for def->src->type !=
VIR_STORAGE_TYPE_VOLUME before setting auth_secret_usage from the
secrettype field.

Luckily the aforementioned commits misdeed would be overwritten by the
call to virStorageTranslateDiskSourcePool