migration: promote debug to info

Some logs we emit in the migration flows
are more suitable as info() than as debug().

This patch is part of the preparation for
the switch to default log level to 'info'
in VDSM.

Change-Id: I57318d02f3c579043413d67c49a070c49c0418b0
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/34354
Reviewed-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>