migration: Fix calculation of bytes_transferred

migration: Fix calculation of bytes_transferred

When a page with all identical bytes is transferred, it is counted
as a full page (TARGET_PAGE_SIZE) although only one byte is actually
sent. Fix this by changing ram_save_block() to return the number of
bytes sent instead of a boolean value. This makes bandwidth
estimation, and consequently downtime estimation, more precise.

Signed-off-by: Pierre Riteau <Pierre.Riteau@irisa.fr>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>