ui/vdagent: core infrastructure

ui/vdagent: core infrastructure

The vdagent protocol allows the guest agent (spice-vdagent) and the
spice client exchange messages to implement features which require
guest cooperation, for example clipboard support.

This is a qemu implementation of the spice client side.  This allows
the spice guest agent talk to qemu directly when not using the spice
protocol.

usage: qemu \
  -chardev qemu-vdagent,id=vdagent \
  -device virtserialport,chardev=vdagent,name=com.redhat.spice.0

This patch adds just the protocol basics: initial handshake and
capability negotiation.  The following patches will add actual
functionality and also add fields to the initially empty
ChardevVDAgent qapi struct.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20210519053940.1888907-1-kraxel@redhat.com
Message-Id: <20210519053940.1888907-5-kraxel@redhat.com>