virtproxy: integration, add a virtproxy chardev
This allows us to create a virtproxy instance via a chardev. It can now
be created with something like:
qemu -chardev virtproxy,id=vp1 \
-device virtio-serial \
-device virtserialport,chardev=vp1
In the future the ability to add oforwards/iforwards in the command-line
invocation and the monitor will be added. For now we leave it to users
of virtproxy (currently only virtagent) to set up the forwarding
sockets/ports they need via direct virtproxy API calls.