virsh_setmem: fix the config file for old_libvirt_test

The domain name and memory size have to be defined to PASS the test
on newer libvirt.
virttest.virsh: change the order of creating the setmem command line

It is possible to call 'virsh setmem --domain test 524288' but the current
code create 'virsh setmem 524288 --domain test' for the right parameters.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>