bdrv_io_plug/bdrv_io_unplug take care of keeping a nesting count,
so change s->plugged to just a bool.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Message-Id: <20180813144320.12382-2-pbonzini@redhat.com>
Signed-off-by: NFam Zheng <famz@redhat.com>