block: prepare bdrv_reopen_multiple to release AioContext

After the next patch bdrv_drain_all will have to be called without holding any
AioContext.  Prepare to do this by adding an AioContext argument to
bdrv_reopen_multiple.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <1477565348-5458-15-git-send-email-pbonzini@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>