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 sFlow VM metrics.

A metric is determined to be an sFlow VM metric when tmax is 60, dmax is 600,
and the metric name contains character '.' followed by character 'v'. The tmax
and dmax values are defined in gmond/sflow.c.

Ex: xenvm1.vcpu_num is an sFlow VM metric, vnode_cpu_num is not.