LUSetInstanceParam: don't assume memory is integer

LUSetInstanceParam currently assumes that the 'memory' value of a
call_instance_info result is an integer, while the rest of the code
explicitely converts it to int(). Converting it to int works around a
bug which prevents changing the memory allocation of a live instance if
the remote call returns the memory in string format.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>