block: convert bdrv_drain_all_begin() to AIO_WAIT_WHILE_UNLOCKED()

Since the AioContext argument was already NULL, AIO_WAIT_WHILE() was
never going to unlock the AioContext. Therefore it is possible to
replace AIO_WAIT_WHILE() with AIO_WAIT_WHILE_UNLOCKED().

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20230309190855.414275-5-stefanha@redhat.com>
Reviewed-by: Wilfred Mallawa <wilfred.mallawa@wdc.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>