virt: Add memory slots for VMXMLBase class in libvirt_xml module.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
virt: Add get_max_mem() for VM class in libvirt_vm module.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
virt: Add setmaxmem() for virsh module.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
libvirt: Add test virsh setmaxmem

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