Add virsh node-memory-tune commands function wrapper

Signed-off-by: Alex Jia <ajia@redhat.com>
Add node memory tuning test configuration and cases

Signed-off-by: Alex Jia <ajia@redhat.com>
libvirt.tests: Fix virsh node-memory-tune testcases.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>