Dealing with unsupported API calls

[fabianfreyer]

Not all libvirt drivers expose the API calls used by the exporter, and the exporter should handle those cases gracefully.

Steps to reproduce

• Using libvirt_exporter with the bhyve driver using libvirt_exporter -libvirt.uri bhyve:///system.
• visit http://hostname:9177

Expected behaviour

Metrics not exposed by libvirt due to non-supported API calls are not exported.

Actual behaviour

Only some metrics of only the first domain are exposed.
The exporter's stdout states the following error message:

2018/08/27 20:23:22 Failed to scrape metrics: virError(Code=3, Domain=20, Message='this function is not supported by the connection driver: virDomainBlockStats')

[beddari]

Conditionals to support this would likely be welcome, judged by the responses from the maintainers in general.