augment info migrate with page status

augment info migrate with page status

This patch augments info migrate output with status about:
* ram bytes remaining
* ram bytes transferred
* ram bytes total

This should be enough for management tools to realize
whether or not there is progress in migration. We can
add more information later on, if the need arrives

[v2: fixes bytes_transferred type]

Signed-off-by: Glauber Costa <glommer@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>