virSecuritySELinuxTransactionCommit: Return -1 if no transaction is set

virSecuritySELinuxTransactionCommit: Return -1 if no transaction is set

Return -1 and report an error message if no transaction is set and
virSecuritySELinuxTransactionCommit is called.

The function description of virSecuritySELinuxTransactionCommit says:

  "Also it is considered as error if there's no transaction set and this
   function is called."

Signed-off-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Reviewed-by: Boris Fiuczynski <fiuczy@linux.ibm.com>