RFE: add an event to report block devices watermark

[Francesco Romani]

Add an event to report if a block device usage exceeds a threshold. The threshold should be configurable with a monitor command. The event should report the affected block device. Additional useful information could be the offset of the highest sector , like in the query-blockstats output.

Rationale for the RFE
Managing applications, like oVirt (http://www.ovirt.org), make extensive use of thin-provisioned disk images.
In order to let the guest run flawlessly and be not unnecessarily paused, oVirt sets a watermark and automatically resized the image once the watermark is reached or exceeded.

In order to detect the mark crossing, the managing application has no choice than aggressively polling the QEMU monitor
using the query-blockstats command. This lead to unnecessary system load, and is made even worse under scale: scenarios
with hunderds of VM are becoming not unusual.

[Francesco Romani (fromani-1)]

patch posted on qemu-devel, reviewd, acked and merged into maintainer's branch:
https://github.com/stefanha/qemu/commit/f050ea639522e9dd7e501ef285a2a12709b8726a

[Thomas Huth (th-huth)]

Upstream here:
https://git.qemu.org/?p=qemu.git;a=commitdiff;h=e2462113b2003085ad16f15e1