Count used RAMBlock pages for migration_dirty_pages

This is a fix for a bug* triggered by a migration after hot unplugging
a few virtio-net NICs, that caused migration never to converge, because
'migration_dirty_pages' is incorrectly initialised.

'migration_dirty_pages' is used as a tally of the number of outstanding
dirty pages, to give the migration code an idea of how much more data
will need to be transferred, and thus whether it can end the iterative
phase.

It was initialised to the total size of the RAMBlock address space,
however hotunplug can leave this space sparse, and hence
migration_dirty_pages ended up too large.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

(* https://bugzilla.redhat.com/show_bug.cgi?id=1074913 )

Signed-off-by: Juan Quintela <quintela@redhat.com>
