Libvirt / Qemu (KVM) does not collect statistics about these either.
On some systems it might even yield a 'internal error' from libvirt
when attempting to gather block statistics from such devices.
For example Ubuntu 16.04 (Xenial) has a issue with this.
Skip them when looping through all devices.
Signed-off-by: Wido den Hollander <wido@widodh.nl>