block-backend: process I/O in the current AioContext

Switch blk_aio_*() APIs over to multi-queue by using
qemu_get_current_aio_context() instead of blk_get_aio_context(). This
change will allow devices to process I/O in multiple IOThreads in the
future.

I audited existing blk_aio_*() callers:
- migration/block.c: blk_mig_lock() protects the data accessed by the
  completion callback.
- The remaining emulated devices and exports run with
  qemu_get_aio_context() == blk_get_aio_context().

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20230912231037.826804-4-stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>