migration: activate TLS thread safety workaround

migration: activate TLS thread safety workaround

When either the postcopy or return path capabilities are
enabled, the migration code will use the primary channel
for bidirectional I/O.

If either of those capabilities are enabled, the migration
code needs to mark the channel as expecting concurrent I/O
in order to activate the thread safety workarounds for
GNUTLS bug 1717

Closes: https://gitlab.com/qemu-project/qemu/-/issues/1937
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/qemu-devel/20250718150514.2635338-4-berrange@redhat.com
Signed-off-by: Fabiano Rosas <farosas@suse.de>