Progress bar not always working about virtnbdbackup

[abbbi]

i think the "issue" is that virtnbdbackup sometimes requests pretty big chunks from the NBD Service, if option
-v is used one can see that in the following example a nearly 50 mb chunk is requested from the NBD server.
During this time, until the complete chunk is received the progress bar isnt updated. The update to the progressbar is done once the complete chunk has been written.

So especially if doing a remote backup where the bandwidth is not as good, it might appear as if no progress is happening. Also, if the disk image has big data chunks in sequence, its noticable. Maybe the chunk.write option can
be changed to yield the size back to the caller so the progressbar can be updated more frequently.

[2023-12-10 08:23:36] DEBUG root virtnbdbackup - backupDisk [vda]:  Chunked data read from: start 1048576, length: 52232192
saving disk vda:   0%|

Alternative would be to pass the progressbar object to chunk.write and update its state there