aio: add polling mode to AioContext

The AioContext event loop uses ppoll(2) or epoll_wait(2) to monitor file
descriptors or until a timer expires.  In cases like virtqueues, Linux
AIO, and ThreadPool it is technically possible to wait for events via
polling (i.e. continuously checking for events without blocking).

Polling can be faster than blocking syscalls because file descriptors,
the process scheduler, and system calls are bypassed.

The main disadvantage to polling is that it increases CPU utilization.
In classic polling configuration a full host CPU thread might run at
100% to respond to events as quickly as possible.  This patch implements
a timeout so we fall back to blocking syscalls if polling detects no
activity.  After the timeout no CPU cycles are wasted on polling until
the next event loop iteration.

The run_poll_handlers_begin() and run_poll_handlers_end() trace events
are added to aid performance analysis and troubleshooting.  If you need
to know whether polling mode is being used, trace these events to find
out.

Note that the AioContext is now re-acquired before disabling notify_me
in the non-polling case.  This makes the code cleaner since notify_me
was enabled outside the non-polling AioContext release region.  This
change is correct since it's safe to keep notify_me enabled longer
(disabling is an optimization) but potentially causes unnecessary
event_notifer_set() calls.  I think the chance of performance regression
is small here.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20161201192652.9509-4-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
