tests/qtest/migration: Use out-of-band execution for migrate-recover

tests/qtest/migration: Use out-of-band execution for migrate-recover

In real use cases, the migrate-recover command requires out-of-band
execution, because the thread processing normal commands is blocked by a
page fault in the guest memory. With this change, the tests will be
closer to real use cases and could help detect regressions and other
bugs in migration recovery.

Signed-off-by: Juraj Marcin <jmarcin@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>