blockjob: add block_job_get_aio_context()

Add a helper function to document why block jobs sometimes run in the
QEMU main loop and to avoid code duplication in a following patch.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-id: 1466096189-6477-5-git-send-email-stefanha@redhat.com
