nodedev: report errors about missing integer properties

nodedev: report errors about missing integer properties

The helper methods for getting integer properties ignore a missing
property setting its value to zero. This lack of error reporting
resulted in missing the regression handling hotplug of USB devices
with the vendor and model IDs getting set to zero silently.

The few callers which relied on this silent defaulting have been fixed,
so now we can report fatal errors immediately.

Reviewed-by: Laine Stump <laine@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>