debug: avoid null dereference on uuid lookup api

debug: avoid null dereference on uuid lookup api

Detected by Coverity.  Commit a98d8f0d tried to make uuid debugging
more robust, but missed some APIs.  And on the APIs that it visited,
the mere act of preparing the debug message ends up dereferencing
uuid prior to the null check.  Which means the APIs which are supposed
to gracefully reject NULL arguments now end up with SIGSEGV.

* src/libvirt.c (VIR_UUID_DEBUG): New macro.
(virDomainLookupByUUID, virDomainLookupByUUIDString)
(virNetworkLookupByUUID, virNetworkLookupByUUIDString)
(virStoragePoolLookupByUUID, virStoragePoolLookupByUUIDString)
(virSecretLookupByUUID, virSecretLookupByUUIDString)
(virNWFilterLookupByUUID, virNWFilterLookupByUUIDString): Avoid
null dereference.