Expose latency histograms via 'virConnectGetAllDomainStats'

Expose latency histograms via 'virConnectGetAllDomainStats'

Add documentation and constants for constructing the stats field names
for latency histograms and expose them in the qemu driver:

Example:

  block.1.latency_histogram.read.bin.count=9
  block.1.latency_histogram.read.bin.0.start=0
  block.1.latency_histogram.read.bin.0.value=0
  block.1.latency_histogram.read.bin.1.start=10
  block.1.latency_histogram.read.bin.1.value=0
  block.1.latency_histogram.read.bin.2.start=100
  block.1.latency_histogram.read.bin.2.value=0
  block.1.latency_histogram.read.bin.3.start=1000
  block.1.latency_histogram.read.bin.3.value=1047
  block.1.latency_histogram.read.bin.4.start=10000
  block.1.latency_histogram.read.bin.4.value=2131
  block.1.latency_histogram.read.bin.5.start=100000
  block.1.latency_histogram.read.bin.5.value=0
  block.1.latency_histogram.read.bin.6.start=1000000
  block.1.latency_histogram.read.bin.6.value=0
  block.1.latency_histogram.read.bin.7.start=10000000
  block.1.latency_histogram.read.bin.7.value=0
  block.1.latency_histogram.read.bin.8.start=100000000
  block.1.latency_histogram.read.bin.8.value=0

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