Add option to prevent gmetad from generating summaries for sFlow VM metrics

This patch adds a new "unsummarized_sflow_vm_metrics" config option to gmetad
to prevent generating summaries for metrics that appears to be sFlow VM metrics.

It doesn't really make sense to generate summaries for VM metrics, since the VM metrics are prefixed by the VM name / ID, ex. vm1.vdisk_capacity, vm2.vdisk_capacity, vm3.vdisk_capacity, etc.

A metric is determined to be an sFlow VM metric when the metric name contains
character '.' followed by character 'v'.

Ex: xenvm1.vcpu_num is an sFlow VM metric, vnode_cpu_num is not.

I'm not entirely satisfied with how the metrics are determined to be sFlow VM metric. I've though of looking at the metric TITLE and DESC extended attribute but haven't found a way to get that information (matching that the VM name ex. xenvm1 in the previous example match the beginning of TITLE or DESC). There is a risk of metrics being inappropriately flagged as sFlow VM metrics (ex. a metric that would be named apache.visitors would match).

Comments and feedbacks are more than welcome.

Simon