libxl, minios: stubdom console based save/restore

libxl, minios: stubdom console based save/restore

Add two "special" PV consoles to stubdoms that are going to be used
to send and receive the qemu-xen save files on save/restore.

Use the second PV console to send the qemu-xen save file and the third
PV console to receive the qemu-xen save file on restore.

Fix the console shutdown function free_consfront that is called when the
qemu save file is closed.

Stubdom save/restore is still broken with xend.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>