migration: export parameters to props

migration: export parameters to props

Export migration parameters to qdev properties. Then we can use, for
example:

  -global migration.x-cpu-throttle-initial=xxx

To specify migration parameters during init.

Prefix "x-" is appended for each parameter exported to show that this is
not a stable interface, and only for debugging/testing purpose.

Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <1500349150-13240-3-git-send-email-peterx@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>