ioemu: drop reschedule of handle_buffered_io on I/O reqs

ioemu: drop reschedule of handle_buffered_io on I/O reqs

Rescheduling the call to handle_buffered_io costs the system call to
qemu_get_clock(), which is very expensive considering the the latency
we would like to achieve for I/O reqs, so we should avoid it.

That means that handle_buffered_io may be called as often as every
0.1s, but that's not so costly.

Signed-off-by: Samuel Thibault <samuel.thibault@eu.citrix.com>