virtio-blk: Allow config-wce in dataplane

Dataplane now uses block layer. Protect bdrv_set_enable_write_cache with
aio_context_acquire and aio_context_release, so we can enable config-wce
to allow guest to modify the write cache online.

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
