iotests: Add test for QMP event rates

iotests: Add test for QMP event rates

This test verifies that the rate-limited QMP events are emitted at a
maximum rate of 1 per second as defined in monitor_qapi_event_conf in
monitor.c

It also checks that QUORUM_REPORT_BAD events generated from different
nodes are kept in separate queues so they don't mask each other.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 0dbd3ee88a59a6363042ad81cfb345037bfbf612.1457610443.git.berto@igalia.com
[mreitz@redhat.com: Renamed test from 146 to 148]
Signed-off-by: Max Reitz <mreitz@redhat.com>