tests/qtest/migration: Replace connect_uri and move migrate_get_socket_address inside...

tests/qtest/migration: Replace connect_uri and move migrate_get_socket_address inside migrate_qmp

Move the calls to migrate_get_socket_address() into migrate_qmp().
Get rid of connect_uri and replace it with args->connect_uri only
because 'to' object will help to generate connect_uri with the
correct port number.

Signed-off-by: Het Gala <het.gala@nutanix.com>
Suggested-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20240312202634.63349-3-het.gala@nutanix.com
Signed-off-by: Peter Xu <peterx@redhat.com>