qemu-iotests: expand test 093 to support group throttling

This patch improves the test by attaching a different number of drives
to the VM and putting them in the same throttling group. The test
verifies that the I/O is evenly distributed among all members of the
group, and that the limits are enforced.

By default the test is repeated 3 times with 1, 2 and 3 drives, but
the maximum number of simultaneous drives is configurable.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 513df1da5c658878191b579ebcddd985adcd4122.1433779731.git.berto@igalia.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
