vdagentd: Do endian swapping.

This allows running big endian and little endian guest side by side using
cut & paste between them.

There is a general design idea that swapping should come as close to
virtio_read/virtio_write as possible. In particular, the protocol
between vdagent and vdagentd is guest-specific and in native endian.
With muliple layers of headers this is a bit tricky. A few message types
have to be swapped fully before passing through vdagentd.

Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Signed-off-by: Victor Toso <me@victortoso.com>