Remote driver needs to make sure the driver lock is released before
entering client IO loop as that may block indefinitely in poll(). As a
direct consequence of not following this in stream APIs, tunneled
migration to a destination host which becomes non-responding may block
qemu driver. Luckily, if keepalive is turned for p2p migrations, both
remote and qemu drivers will get automagically unblocked after keepalive
timeout.