virt: sampling: consolidate disk statistics

We used to have two different sampling, and thus two
libvirt calls per sampling, per disk.
One sampling gathers basic disk data and R/W rate;
the other one gathers the disk latency.

Most often, as the default values are unchanged, the
configuration for these sampling is identical, so it is safe
and cheaper to read all the needed information in one go,
saving one libvirt call without losing any information.

These sampling uses two different libvirt calls.
sampleDisk, which gathers the disk I/O rate, uses the older
virDomainGetBlockStats call;
sampleDiskLatency, which gathers disk latency, uses the newer
and more flexible virDomainGetBlockStatsFlags;

Inside libvirt, these two calls are implemented on top
of the same QEMU monitor function (qemuMonitorGetBlockStatsInfo).
The newer call returns all the data gathered by the QEMU monitor,
while the older call returns just a subset of it.

Other differences in the API exists, like the return type
and the presence of the flags, but those can safely be considered
of no interest for VDSM.

Change-Id: I0dabd079f81270c7099c74469a18f8b23c97cc8c
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/29953
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>