ProcFSTools: fix read_meminfo without KSM

ProcFSTools: fix read_meminfo without KSM

on kernel configs where KSM is not enabled (e.g. an openstack instance at
OVH) the file /sys/kernel/mm/ksm/pages_sharing does not exist.
In that case there is no memory shared so assuming 0 is reasonable.

While this is not the case with our shipped kernel, and thus will probably
not happen on a PVE installation, PMG can run quite happily with a different
kernel (the reporter had this on an openstack instance at OVH).

a quick grep through our codebase showed only the API2::Nodes::Nodeinfo::status
call as user of the memshared property.

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>