improve efficiency of image transfer during migration
improve efficiency of image transfer during migration

This reduces time to transfer a qcow2 image
with a virtual size of 10G, over GigE,
from about 7 minutes to about 30 seconds.

There are multiple inefficiencies in the existing process.
Taking an example of a qcow2 image with 10G virtual size,
the process was:

  qcow2 -> raw -> read -> send -> write -> qcow2

qcow2 to raw takes 20s,
transfer of the resultant 10G is another 4m9s, and
conversion back to qcow takes 2m33s.
I.E. a total of about 7 minutes.

So instead we try to avoid the initial qcow2 to raw
conversion completely, which results in the whole
process completing in about 30s, in the common
case where no conversion to raw is done on the destination.

We also optimize the case where the source qcow2
image doesn't have a backing file, and then directly
copy the source image without merging a backing file.

Note this will also improve the situation when
resizing/migrating within the same host as
needles conversions are avoided in that case too.

We also optimize the case where raw images are being used
by trying to use `rsync -Sz` rather than `scp`.
That compresses runs of zeros and create sparse destination files.
Testing a 10G raw image showed a saving of 30s in transfer time.
Also the network was greatly reduced (corresponding to holes
in the source), as was space usage at the destination.
This gain is limited though by rsync inefficiently reading
all the holes at the source:
https://bugzilla.samba.org/show_bug.cgi?id=8918

Thanks to David Naori <dnaori@redhat.com> for testing and ideas.

Change-Id: I9e87f912ef2717221c244241cda2f1027a4ca66a

n0ano/gantt,n0ano/gantt