virt plugin: support virDomainBlockStatsFlags

We want to export more disk stats, like disk_time.
To do so, we must use the virDomainBlockStatsFlags API, but it
could not be available on all the platforms collectd supports.
To cope with that, we add a minimal layer around the API, and
we use virDomainBlockStatsFlags only if available.

This patch adds the code to use the new API and the fallback code
in the case the platform' libvirt is too old.

Signed-off-by: Francesco Romani <fromani@redhat.com>