avocado.job: Move sysinfo collection to test result object

We found a bug: The sysinfo collection was overwriting the
post information in jobs run through the vm plugin. Fix
the problem by moving the information collection to the
test result object. This is transparent to plugins, one
important requirement of the solution.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #240 from lmr/fix-sysinfo-bug-vm-plugin

avocado.job: Move sysinfo collection to test result object