linux-aio: Cancel BH if not needed

linux-aio uses a BH in order to make sure that the remaining completions
are processed even in nested event loops of completion callbacks in
order to avoid deadlocks.

There is no need, however, to have the BH overhead for the first call
into qemu_laio_completion_bh() or after all pending completions have
already been processed. Therefore, this patch calls directly into
qemu_laio_completion_bh() in qemu_laio_completion_cb() and cancels
the BH after qemu_laio_completion_bh() has processed all pending
completions.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
