LibVMI cache : bug fix and unit test

1. fix the bug : key_128_int() misses the high 32bits of the uint64_t low.
As vmi->page_size is a uint32_t type, we can't assume it the same as uint64_t when using the bitwise NOT of it.

2. unit test for LibVMI cache

Thanks to awsaba's help in fixing and unit testing.