virsh_numatune: Catch errors from XML parser correctly

If the domain XML doesn't contain the <numatune> element then
'strict' mode with all memory nodes is assumed. Tweak the test so that
it will take this into account.
virsh_numatune: Move testing of setting 'strict' NUMA mode for live guest

Changing of the NUMA placement mode for a running guest should fail.
This is true unless the user is trying to set the same mode which is
not considered as an error.

This patch moves the test case of setting 'strict' NUMA placement mode
from negative testing to positive with a running domain where the
configuration can be applied successfully
virsh_numatune: Fix catching of 'KeyError' exception

Remove the parentheses after the exception name to catch it correctly.
virsh_numatune: Skip tests depending on cgconfig on systemd machines

On systemd machines the cgroup subsystem is managed by systemd. This
makes it impossible to test stuff with disabled cgroup support. Skip
the offending tests in such case.