[libvirt] [PATCH 2/5]: Properly set errors on unknown format types

Because of my patch last week that converted the various virStorage*FromString and virStorage*ToString implementations to the generic VIR_ENUM_IMPL, there were a couple of places that didn't properly set errors when they failed. This patch fixes these places up. It also adds an additional check in virEnumFromString(), so that if a NULL type is passed in, it fails instead of SEGV'ing. Signed-off-by: Chris Lalancette <clalance(a)redhat.com&gt;