qemu: Fix the wrong expression

qemu: Fix the wrong expression

Wrong use of the parentheses causes "rc" always having a boolean value,
either "1" or "0", and thus we can't get the detailed error message
when it fails:

Before (I only have 1 node):
% virsh numatune f18 --nodeset 12
error: Unable to change numa parameters
error: unable to set numa tunable: Unknown error -1

After:
virsh numatune f18 --nodeset 12
error: Unable to change numa parameters
error: unable to set numa tunable: Invalid argument