If a client is handling multiple SpiceFileTransferTasks at one time,
it's not currently possible to provide a single overall progress to the
user. The only information that the client can get is the percentage
progress. This patch adds two new properties:
 - total-bytes: the size of the file transfer task in bytes
 - transferred-bytes: the number of bytes already transferred

This allows a client UI to calculate the combined progress for all
ongoing transfer tasks and present it to the user. Two convenience
functions were added to retrieve these values:
 - spice_file_transfer_task_get_total_bytes()
 - spice_file_transfer_task_get_transferred_bytes()