mirror: follow AioContext change gracefully

Add block_job_pause_point() calls to mark quiescent points and make sure
to complete in-flight requests when switching AioContexts.

This patch solves undefined behavior in the mirror block job when the
BDS AioContext is changed by dataplane.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-id: 1466096189-6477-8-git-send-email-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
