qemu-io: check registered fds in command_loop()

Some block drivers use an aio handler and do I/O completion routines
in it.  However, the handler is not invoked if we only do
aio_read/write, because registered fds are not checked at all.

This patch registers an aio handler of STDIO to checks whether we can
read a command without blocking, and calls qemu_aio_wait() in
command_loop().  Any other handlers can be invoked when user input is
idle.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
