tests: optparse: Use --config with the setmaxmem command

tests: optparse: Use --config with the setmaxmem command

The virsh-optparse test broke after commit 6ac402c456a because it
always assumed the max memory limit can be adjusted on a running domain
which used to be the case in the old code.
This is only a hot fix for the CI build. The proper fix here is to
re-write the whole test in a self-test/unit-test manner where we only
test virsh's ability to parse various values, not running actual
commands.

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Acked-by: Peter Krempa <pkrempa@redhat.com>