CreateInstance: separate the reading of the export

We move the reading of the export to a separate function, to simplify
CheckPrepreq and also read it earlier. This will allow building the
missing opcode parameters from the export information, instead of
requiring all of them on the command line.

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