virAuth: Don't require virConnectPtr to retrieve authentication creds

virAuth: Don't require virConnectPtr to retrieve authentication creds

Previously a connection object was required to retrieve the auth
credentials. This patch adds the option to call the retrieval functions
only using the connection URI or path to the configuration file. This
will allow to use this toolkit to request passwords for ssh
authentication in the libssh2 connection driver.

Changes:
*virAuthGetConfigFilePathURI(): use URI to retrieve the config file path
*virAuthGetCredential(): Remove the need to propagate conn object

virAuthGetPasswordPath():
*virAuthGetUsernamePath(): New functions, that use config file path
                           instead of conn object