libvirt pmda: use error codes that do not generate log warnings
libvirt pmda: use error codes that do not generate log warnings

For lost-connection style errors, we tend to use PM_ERR_AGAIN
and not PM_ERR_NOTCONN (used by pmcd to signify agent down) -
this squashes libpcp_pmda error reports into the PMDA logfile.

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp