Add soft dependency on libvirt's metadata support to stats collector

There is an issue with libvirt on CentOS 6. The available version does
not support part of the metadata API. This generates an error message
every 15 seconds.

This patch workarounds this by logging the error only once.

Revert once libvirt supports the xml element metadata on CentOS.

See bug: https://bugzilla.redhat.com/show_bug.cgi?id=1115039

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1114968
Change-Id: I3d38d3aff3bda42dfaafa87bab9fa5983fb4b9a8
Signed-off-by: Martin Sivak <msivak@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/29504
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Michal Skrivanek <michal.skrivanek@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
(cherry picked from commit 935123db785d0a848cfeae5e9cf8cc75ad598701)
Reviewed-on: http://gerrit.ovirt.org/29788