Fix up inconsistent virsh option error reporting.

Fix up inconsistent virsh option error reporting.

The virsh option error reporting was not being used
consistently; some commands would spit out errors on
missing required options while others would just silently fail.
However, vshCommandOptString knows which ones are required
and which ones aren't, so make it spit out an error where
appropriate.  The rest of the patch is just cleaning up
the uses of vshCommandOptString to deal with the new error
reporting.

Signed-off-by: Chris Lalancette <clalance@redhat.com>