Graceful handling of missing /proc/vmstat

[dpavlenkov]

Joyent's LX brand zones don't have /proc/vmstat TritonDataCenter/illumos-joyent#33 and I don't think they will in near future. Would like to not throw error on https://github.com/giampaolo/psutil/blob/master/psutil/_pslinux.py#L260 but try alternative ways of getting swap page in/out values.

[giampaolo]

On what linux kernel is this missing? It is reasonable to catch IOError if the file is not there and set swap in/out values to 0. Feel free to provide a PR if you want.