Extend vmi_pid_t to 128-bit so that it can be casted to be process_conte...

...xt_t which contains both a 32-bit pid and 64-bit dtb.

This PR allows one to safely cast process_context_t to vmi_pid_t to be used in the various *va functions available in LibVMI, while also being backwards compatibly where the vmi_pid_t was just an integer.

By casting a process_context_t to vmi_pid_t we can pass a dtb directly for libvmi to be used, instead of libvmi using its internal process list walking to determine the dtb from the pid. For this, the process_context_t has to specify a negative (invalid) pid, which triggers libvmi to use the dtb value in the context for the VA translation.