Introduce virStorageAuthDef and friends.  Future patches will merge/utilize
their view of storage source/pool auth/secret definitions.

New API's include:
    virStorageAuthDefParse:  Parse the "<auth/>" XML data for either the
                             domain disk or storage pool returning a
                             virStorageAuthDefPtr
    virStorageAuthDefCopy:   Copy a virStorageAuthDefPtr - to be used by
                             the qemuTranslateDiskSourcePoolAuth when it
                             copies storage pool auth data into domain
                             disk auth data
    virStorageAuthDefFormat: Common output of the "<auth" in the domain
                             disk or storage pool XML
    virStorageAuthDefFree:   Free memory associated with virStorageAuthDef

Subsequent patches will utilize the new functions for the domain disk and
storage pools.

Future work in the hostdev pass through can then make use of common data
structures and code.