qemu-iotests: Add 093 for IO throttling

This case utilizes qemu-io command "aio_{read,write} -q" to verify the
effectiveness of IO throttling options.

It's implemented by driving the vm timer from qtest protocol, so the
throttling timers are signaled with determinied time duration. Then we
verify the completed IO requests are within 10% error of bps and iops
limits.

"null" protocol is used as the disk backend so that no actual disk IO is
performed on host, this will make the blockstats much more
deterministic. Both "null-aio" and "null-co" are covered, which is also
a simple cross validation test for the driver code.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 1422586186-9925-6-git-send-email-famz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
