virIndexToDiskName: Make 'idx' unsigned and remove check

virIndexToDiskName: Make 'idx' unsigned and remove check

We can remove the check that 'idx' is negative by forcing callers to
pass unsigned numbers, which they do already or have a check that 'idx'
is positive.

This in turn allows us to remove most return value NULL checks.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>