Document that virNodeGetInfo can return mhz == 0.

Document that virNodeGetInfo can return mhz == 0.

On the s/390x architecture, libvirt may already return 0 in the
node_info->mhz field (see src/nodeinfo.c:linuxNodeInfoCPUPopulate).

We may also want to return this on aarch64 in future, because
calculating the proper value requires SMBIOS, which is not available
on non-server-class systems (specifically on systems which don't
adhere to the SBSA standard).

Therefore this change documents the existing behaviour and provides a
valid path for aarch64.

Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
Bug-URL: https://bugzilla.redhat.com/1206353