Generalize getting xids from /proc/virtual directory.
Generalize getting xids from /proc/virtual directory.

This change combines common code for listing the xids found in the
/proc/virtual directory. No new tests are needed because the
get_vserver_xids method is called unconditionally by other methods that
are tested.

stephen-soltesz/collectd-mlab,m-lab/collectd-mlab,m-lab/collectd-mlab,stephen-soltesz/collectd-mlab