src: adopt to VIR_DRV_SUPPORTS_FEATURE return -1

src: adopt to VIR_DRV_SUPPORTS_FEATURE return -1

Otherwise in some places we can mistakenly report 'unsupported' error instead
of root cause. So let's handle root cause explicitly from the macro.

Signed-off-by: Nikolay Shirokovskiy <nshirokovskiy@virtuozzo.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>