pmdagfs2: Improvements for distributions without all gfs2 trace-points

Improved error handling for distributions that do not currently have
support for the full set of GFS2 trace-points.

Additional cases are used within all metrics fetch functions to check
for a pre-determined value which is set if the trace-point does not
exist on the system. This sets the metric values as "No Value(s)
Available" and is a cleaner output than zeros in these cases.

Also a little spring cleaning with some of the coding conventions.

Additional: Fixed issue where direct lookup without bounds check
could result in invalid array position being read.
pmdagfs2: qa improvements to guarding and compatible setups

Have reduced the number of tests run on qa/654 so that it can be run
on most installations with gfs2-utils present. Checked against Fedora
16-19, Ubuntu 13.04, Centos 6.4 and Debian 7 with Debian 7 being the
most restrictive through only having support for gfs2.glock metrics.

Included additional checks in qa/655 to test for the mounting of debugfs
and to attempt to mount it in the event that it is already not. Have
re-added added additional comments on suggestions how to quickly setup a
configuration for testing 655 in qemu-kvm using the cbox application.

Added additional functions to common.gfs2 that test for the availability of
mkfs.gfs2 on the test system (part of gfs2-utils) as well as attempting to
mount debugfs on distributions that do not have it mounted by default.

qa/654.out reflects the changes made to qa/654 in order to verify a correct
run.

Additional: Re-added non-deterministic view of loopN on tests in qa/654 and
added pmstore checking in qa/655 to ensure the functionality is checked in
future (testing gap).