Commit ed2aec4867f0d5f5de496bb765347b5d0cfe113d changed the return
value of aio_ctx_prepare from false to true when only idle bottom
halves are available.  This broke PC old-style DMA, which uses them.
Fix this by making aio_ctx_prepare return true only when non-idle
bottom halves are scheduled to run.

Reported-by: malc <av1474@comtv.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: malc <av1474@comtv.ru>