virt unit tests: Add valgrind.suppress with libnl1 leak definition

virt unit tests: Add valgrind.suppress with libnl1 leak definition

Libnl-1 is used by libvirt on older systems (e.g. debian wheezy)
and unfortunately it has memory leaks that cannot be fixed from
collectd side - virt unit tests were failing on platforms with
libnl1 because of those leaks.
Valgrind.suppress file is used to pass definitions for leaks that
cannot be fixed and now virt unit tests can be run without any
problems on every platform - memory leaks are still validated and
the only difference is that libnl1 leaks are ignored during
valgrind check.

Change-Id: I5fc423402f7b89e4e43fb896f73cba89eea36226
Signed-off-by: Radoslaw Jablonski <radoslawx.jablonski@intel.com>