tests/migration: Setup pre-listened cpr.sock to remove race-condition.

tests/migration: Setup pre-listened cpr.sock to remove race-condition.

When the source VM attempts to connect to the destination VM's Unix
domain socket (cpr.sock) during a cpr-transfer test, race conditions can
occur if the socket file isn't ready. This can lead to connection
failures when running tests.

This patch creates and listens on the socket in advance, and passes the
pre-listened FD directly. This avoids timing issues and improves the
reliability of CPR tests.

Reviewed-by: Jason J. Herne <jjherne@linux.ibm.com>
Signed-off-by: Jaehoon Kim <jhkim@linux.ibm.com>
Reviewed-by: Steve Sistare <steven.sistare@oracle.com>
Link: https://lore.kernel.org/r/20250611205610.147008-2-jhkim@linux.ibm.com
[peterx: null-initialize opts_target, per Steve]
Signed-off-by: Peter Xu <peterx@redhat.com>