virsh: Make self-test failures noisy

virsh: Make self-test failures noisy

In local testing, I accidentally introduced a self-test failure,
and spent way too much time debugging it. Make sure the testsuite
log includes some hint as to why command option validation failed.
Lone exception: allocation failure is unlikely during self-test,
and if it happens, we are better off asserting (vsh.c can do this,
even if libvirt.so cannot).

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>
Acked-by: Peter Krempa <pkrempa@redhat.com>