CreateInstance: Move some code to CheckArguments

ExpandNames holds too much non-locking code (first LU to be converted to
ExpandNames, and we didn't have CheckArguments at that poin), and this
patch moves the checks that are lock-independent to CheckArguments.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>