Implement acceptance test for PXE booted vm resources

[ddelnano]

In #155 I added PXE support to the VM resource. I wanted to have an acceptance test for this feature, however, my main testing environment's PXE menu defaults to "local boot". This means that the VM in the current test starts up and then fails to boot because it doesn't have an OS installed on its disk.

I wasn't able to find a way to automate selecting an option on the client (guest VM) side. It seems the boot menu is driven entirely on the PXE server. One way to work around this is to have the terraform acceptance test use a websocket based VNC client to navigate the PXE boot menu (pressing down arrow (<down>) and then selecting an option with <enter>). Most of this functionality already exists in the packer-plugin-xenserver repo, however, it still is slightly more of a time commitment to implement.

This GitHub issue will serve as a placeholder for returning to this work to unblock shipping the initial version of #155.