VPI: add support for detecting ports and their directions

[kammoh]

Description
The changes enable vpi_get(vpiType, handle_to_port) to return vpiPort if the signal is a port. It also enables vpi_get(vpiDirection, handle_to_port) to return correct port directions(vpiInput, vpiOutput, or vpiInout).
The changes does not change behavior for other types of signals.

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[tgingold]

This is OK, but some builds are failing due to trailing spaces. Can you remove them ?
You can also consider 'buffer' ports to be outputs.

[kammoh]

@tgingold should ports with VhpiBufferMode be treated as vpiOutput or vpiInout instead of vpiNoDirection?

[kammoh]

@tgingold I fixed the indentation issue but travis is still failing and I can't tell the reason

[eine]

@xnorme it is failing because of gna/issue450. It expects the output in disptree.ref. Instead, this is generated. As you can see, three lines are missing after Simple module name (vpiName): disptree. Does it make sense to you?

[kammoh]

Thanks @1138-4eb. I had missed the point that a vpiPort is (or could be?) a vpiNet as well. Will try to push a fix.

[tgingold]

Thanks!

Bonus point if buffer ports are also considered as vpiOutput.