util: Sanitize ATTRIBUTE_NONNULL use in viriscsi.h

util: Sanitize ATTRIBUTE_NONNULL use in viriscsi.h

Some of the function attributes marked as nonnull actually explicitly
handle the arguments for NULL. All changed functions handle missing
"initiatoriqn" argument well and virISCSIScanTargets also handles well
if the return pointers are missing. Remove some of the liberaly used
ATTRIBUTE_NONNULLs as coverity and possibly other compilers that honor
the attribute fail to compile the code.

Flaw introduced in commit 5e1d5dde