* add open_perf_event api for reading perf counters

Though the rewriter table method existed, we were not yet opening up the
perf counter in the proper way for it to be read.

Introduce a c function to enable attaching perf counters to cpu-indexed
table slots. The python side opens, assigns, then immediately closes the
fd. Only the kernel keeps a reference, so closing the table or deleting
the entry will be sufficient to free up the fd when finished.
Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>

* Skip perf array hw counter test if unsupported

Hardware counters are not available in all places (some VM
environments), so gracefully skip on the particular error condition.
Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>