LXC: show used memory as 0 when domain is not active

LXC: show used memory as 0 when domain is not active

Before:
virsh # dominfo chx3
State:          shut off
Max memory:     92160 KiB
Used memory:    92160 KiB

After:
virsh # dominfo container1
State:          shut off
Max memory:     92160 KiB
Used memory:    0 KiB

Similar to qemu cases.

Signed-off-by: Chen Hanxiao <chenhanxiao@cn.fujitsu.com>