blockjob: add AioContext attached callback

Block jobs that use additional BDSes or event loop resources need a
callback to get their affairs in order when the AioContext is switched.

Simple block jobs don't need an attach callback, they automatically work
thanks to the generic attach/detach notifiers that this patch adds.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-id: 1466096189-6477-7-git-send-email-stefanha@redhat.com
