migration: introduce migrate_params_check()

Helper to check the parameters. Abstracted from
qmp_migrate_set_parameters().

Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <1500349150-13240-5-git-send-email-peterx@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
