migration: Add migrate_use_tls() helper

migration: Add migrate_use_tls() helper

A lot of places check parameters.tls_creds in order to evaluate if TLS is
in use, and sometimes call migrate_get_current() just for that test.

Add new helper function migrate_use_tls() in order to simplify testing
for TLS usage.

Signed-off-by: Leonardo Bras <leobras@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20220513062836.965425-6-leobras@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>