qga: Move virtio property inside GAChannel

virtio property is a channel property so move it to its real place
for not cluttering GAState structure.
configure: Add --enable/disable-guest-session option

This option builds session-mode support into QEMU Guest Agent which is
used for clipboard synchronization.

If guest agent is not enabled, session support is automatically off.
Also, if --enable-gtk --with-gtkabi=3.0 is not passed, session support
is disabled since it requires GTK libraries for clipboard ability.