VNC port numbers should be absolute.

#### Overview of the Issue

VNC port should be absolute, not relative.

#### Reproduction Steps

N/A

### Packer version

Packer v1.6.6

When packer starts it tells the VNC port it's working on:

BASE-opensuse15_3-libvirt: The VM will be run headless, without a GUI. If you want to
BASE-opensuse15_3-libvirt: view the screen of the VM, connect via VNC without a password to
BASE-opensuse15_3-libvirt: vnc://0.0.0.0:82

Where is this _relative_ port number specification defined? Port numbers are (almost?) ubiquitously absolute. Some (many, all?) VNC clients use absolute port numbers and when given a relative one try to connect to that port, not that port+5900.

Search the Packer Go source for the message that reports the VNC endpoint and trace how its port value is produced. Check existing tests around VNC or port reporting, if present. Done means the reported port is absolute and the relevant behavior is covered by a test.

Written by the indexing model from the issue text.