Add 'type' to agent_fmt

Add 'type' to agent_fmt

This adds an extra field to agent_fmt that specifes the type of guest
agent connection to use. Currently there is no choice, and defaults to
virtio-serial. Since qemu-ga also runs over isa-serial, this allows OSes
such as NetBSD and OpenBSD, which do not have support for virtio-serial,
to run a qemu-ga.

This is an optional field, which leaves the default as virtio-serial. As
it doesn't change the default, it will require no change to older
configuration files.

Signed-off-by: Matt Dunwoodie <ncon@noconroy.net>