virsh.setmem: Update case for inactive dominfo change

In the following patch inactive domain used memory is shown as 0 in
`virsh dominfo`.
https://www.redhat.com/archives/libvir-list/2014-July/msg00887.html

This patch fix setmem according to this change.

Signed-off-by: Hao Liu <hliu@redhat.com>
Merge pull request #309 from Hao-Liu/fix_setmem

virsh.setmem: Update case for inactive dominfo change