Open VDI Port when spice_use_vdagent is enabled

To use the features spice agent provides, a dedicated virtio-serial
channel must be created in qemu-kvm. The communication between the agent
and the other spice components takes place over this channel.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Andrea Spadaccini <spadaccio@google.com>