driver: fix handling of error return from finding resource

driver: fix handling of error return from finding resource

The virFileFindResource method merely builds up the expected fully
qualified path to the resource. It does not actually check if it exists
on disk. The loadable module callers were mistakenly thinking a NULL
indicates the file doesn't exist on disk, whereas it in fact indicates
an out of memory error.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>