So far progress reporting only works for the first block device. Fix
this by keeping an overall sum of sectors to be migratated, calculating
the sum of all processed sectors, and finally basing the progress
display on those values.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>