migration: yay, buffering is gone

migration: yay, buffering is gone

Buffering was needed because blocking writes could take a long time
and starve other threads seeking to grab the big QEMU mutex.

Now that all writes (except within _complete callbacks) are done
outside the big QEMU mutex, we do not need buffering at all.

Reviewed-by: Orit Wasserman <[email protected]>
Reviewed-by: Juan Quintela <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>
Signed-off-by: Juan Quintela <[email protected]>