block: Fix BB AIOCB AioContext without BDS

Fix the BlockBackend's AIOCB AioContext for aborting AIO in case there
is no BDS. If there is no implementation of AIOCBInfo::get_aio_context()
the AioContext is derived from the BDS the AIOCB belongs to. If that BDS
is NULL (because it has been removed from the BB) this will not work.

This patch makes blk_get_aio_context() fall back to the main loop
context if the BDS pointer is NULL and implements
AIOCBInfo::get_aio_context() (blk_aiocb_get_aio_context()) which invokes
blk_get_aio_context().

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
