BUG/MEDIUM: quic-be: do not launch the connection migration process

BUG/MEDIUM: quic-be: do not launch the connection migration process

At this time the connection migration is not supported by QUIC backends.
This patch prevents this process to be launched for connections to QUIC backends.

Furthermore, the connection migration process could be started systematically
when connecting a backend to INADDR_ANY, leading to crashes into qc_handle_conn_migration()
(when referencing qc->li).

Thank you to @InputOutputZ for having reported this issue in GH #3178.

This patch simply checks the connection type (listener or not) before checking if
a connection migration must be started.

No need to backport because support for QUIC backends is available from 3.3.