util: Propagate numad failures correctly

util: Propagate numad failures correctly

Right now, if numad fails, we raise an error but return an
empty string to the caller instead of a NULL pointer, which
means processing will continue and the user will see

  # virsh start guest
  error: Failed to start domain guest
  error: invalid argument: Failed to parse bitmap ''

instead of a more reasonable

  # virsh start guest
  error: Failed to start domain guest
  error: operation failed: Failed to query numad for the advisory nodeset

Make sure the user gets a better error message.

https://bugzilla.redhat.com/show_bug.cgi?id=1716387

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>