vdagentd: Do endian swapping.

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>
---
v2:
 - introduce helper functions to swap (a portion of) a message wholesale
 - pollute fewer places with swapping sometimes at the cost of slightly
   more verbose code
v3:
 - use glib byteswap macros in place of endian.h byteswap macros
 - move variable declaration out of case statement
 - reuse more of existing clipboard code
v4:
 - also use glib byteswap for 64bit swaps
 - use file xfer message structure for swapping size
v5:
 - rebase on top of vdagentd: early return on bad message size