qemu-img ftp/http convert

Converting images with ftp or http as source could be done a lot faster. The way it works now (qemu 1.7.50) is significantly slower than the optimal way. 

FTP - how it works now
1. Connect and login to ftp-server. Ask for size of file.
2. Get a chunk of data using rest+retr
3. Goto step 1 again in a loop until all data is retrieved

FTP - better solution
1. Connect and login to ftp-server. Dont ask for size of file.
2. Retrieve all remaining data
3. Goto step 1 again if disconnected/io error (max NN errors etc)


Http - how it works now
1. Connect to webserver and ask for size of file / http HEAD.
2. Get a chunk of data using http Range.
3. Goto step 1 again in a loop until all data is retrieved.

Http - better solution
1. Connect to webserver.
2. Retrieve all remaining data.
3. Goto step 1 again if disconnected/io error (max NN errors).