qemu: Setup disk latency histograms on startup/hotplug/update

qemu: Setup disk latency histograms on startup/hotplug/update

Setup the histograms on startup and hotplug of devices via
'qemuProcessSetupDiskPropsRuntime' and facilitate update/reset/disable
of histogram collection via 'qemuDomainChangeDiskLive'.

The latter allows to use the update device API to either clear the bins
or select new bin configuration or disable the histogram altogether
without the need for a specific API.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>