tools: Use real async.c instead of stubs

It's wrong to call BHs directly, even in tools. The only operations that
schedule BHs are called in a loop that (indirectly) contains a call to
qemu_bh_poll anyway, so we're not losing the scheduled BHs: Tools either use
synchronous functions, which are guaranteed to have completed (including any
BHs) when they return; or if they use asynchronous functions, they need to call
qemu_aio_wait() or similar functions already today.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
