tests/migration-test: Add a test for postcopy hangs during RECOVER

To do so, create two paired sockets, but make them not providing real data.
Feed those fake sockets to src/dst QEMUs for recovery to let them go into
RECOVER stage without going out.  Test that we can always kick it out and
recover again with the right ports.

This patch is based on Fabiano's version here:

https://lore.kernel.org/r/877cowmdu0.fsf@suse.de



Signed-off-by: Fabiano Rosas <farosas@suse.de>
[peterx: write commit message, remove case 1, fix bugs, and more]
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-ID: <20231017202633.296756-4-peterx@redhat.com>