coroutine-lock: make qemu_co_enter_next thread-safe

qemu_co_queue_next does not need to release and re-acquire the mutex,
because the queued coroutine does not run immediately.  However, this
does not hold for qemu_co_enter_next.  Now that qemu_co_queue_wait
can synchronize (via QemuLockable) with code that is not running in
coroutine context, it's important that code using qemu_co_enter_next
can easily use a standardized locking idiom.

First of all, qemu_co_enter_next must use aio_co_wake to restart the
coroutine.  Second, the function gains a second argument, a QemuLockable*,
and the comments of qemu_co_queue_next and qemu_co_queue_restart_all
are adjusted to clarify the difference.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20180203153935.8056-5-pbonzini@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>

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